Aarch64 codegen quality: handle add-negative-imm as subtract.

We often see patterns like:

```
    mov w2, #0xffff_ffff   // uses ORR with logical immediate form
    add w0, w1, w2
```

which is just `w0 := w1 - 1`. It would be much better to recognize when
the inverse of an immediate will fit in a 12-bit immediate field if the
immediate itself does not, and flip add to subtract (and vice versa), so
we can instead generate:

```
    sub w0, w1, #1
```

We see this pattern in e.g. `bz2`, where this commit makes the following
difference (counting instructions with `perf stat`, filling in the
wasmtime cache first then running again to get just runtime):

pre:

```
        992.762250      task-clock (msec)         #    0.998 CPUs utilized
               109      context-switches          #    0.110 K/sec
                 0      cpu-migrations            #    0.000 K/sec
             5,035      page-faults               #    0.005 M/sec
     3,224,119,134      cycles                    #    3.248 GHz
     4,000,521,171      instructions              #    1.24  insn per cycle
   <not supported>      branches
        27,573,755      branch-misses

       0.995072322 seconds time elapsed
```

post:

```
        993.853850      task-clock (msec)         #    0.998 CPUs utilized
               123      context-switches          #    0.124 K/sec
                 1      cpu-migrations            #    0.001 K/sec
             5,072      page-faults               #    0.005 M/sec
     3,201,278,337      cycles                    #    3.221 GHz
     3,917,061,340      instructions              #    1.22  insn per cycle
   <not supported>      branches
        28,410,633      branch-misses

       0.996008047 seconds time elapsed
```

In other words, a 2.1% reduction in instruction count on `bz2`.

cranelift/codegen/src/isa/aarch64/lower.rs

@@ -424,6 +424,35 @@ pub(crate) fn put_input_in_rse_imm12<C: LowerCtx<I = Inst>>(
     ResultRSEImm12::from_rse(put_input_in_rse(ctx, input, narrow_mode))
 }
 
+/// Like `put_input_in_rse_imm12` above, except is allowed to negate the
+/// argument (assuming a two's-complement representation with the given bit
+/// width) if this allows use of 12-bit immediate. Used to flip `add`s with
+/// negative immediates to `sub`s (and vice-versa).
+pub(crate) fn put_input_in_rse_imm12_maybe_negated<C: LowerCtx<I = Inst>>(
+    ctx: &mut C,
+    input: InsnInput,
+    twos_complement_bits: usize,
+    narrow_mode: NarrowValueMode,
+) -> (ResultRSEImm12, bool) {
+    assert!(twos_complement_bits <= 64);
+    if let Some(imm_value) = input_to_const(ctx, input) {
+        if let Some(i) = Imm12::maybe_from_u64(imm_value) {
+            return (ResultRSEImm12::Imm12(i), false);
+        }
+        let sign_extended =
+            ((imm_value as i64) << (64 - twos_complement_bits)) >> (64 - twos_complement_bits);
+        let inverted = sign_extended.wrapping_neg();
+        if let Some(i) = Imm12::maybe_from_u64(inverted as u64) {
+            return (ResultRSEImm12::Imm12(i), true);
+        }
+    }
+
+    (
+        ResultRSEImm12::from_rse(put_input_in_rse(ctx, input, narrow_mode)),
+        false,
+    )
+}
+
 pub(crate) fn put_input_in_rs_immlogic<C: LowerCtx<I = Inst>>(
     ctx: &mut C,
     input: InsnInput,

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -60,8 +60,17 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             let rn = put_input_in_reg(ctx, inputs[0], NarrowValueMode::None);
             let ty = ty.unwrap();
             if ty_bits(ty) < 128 {
-                let rm = put_input_in_rse_imm12(ctx, inputs[1], NarrowValueMode::None);
-                let alu_op = choose_32_64(ty, ALUOp::Add32, ALUOp::Add64);
+                let (rm, negated) = put_input_in_rse_imm12_maybe_negated(
+                    ctx,
+                    inputs[1],
+                    ty_bits(ty),
+                    NarrowValueMode::None,
+                );
+                let alu_op = if !negated {
+                    choose_32_64(ty, ALUOp::Add32, ALUOp::Add64)
+                } else {
+                    choose_32_64(ty, ALUOp::Sub32, ALUOp::Sub64)
+                };
                 ctx.emit(alu_inst_imm12(alu_op, rd, rn, rm));
             } else {
                 let rm = put_input_in_reg(ctx, inputs[1], NarrowValueMode::None);
@@ -79,8 +88,17 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             let rn = put_input_in_reg(ctx, inputs[0], NarrowValueMode::None);
             let ty = ty.unwrap();
             if ty_bits(ty) < 128 {
-                let rm = put_input_in_rse_imm12(ctx, inputs[1], NarrowValueMode::None);
-                let alu_op = choose_32_64(ty, ALUOp::Sub32, ALUOp::Sub64);
+                let (rm, negated) = put_input_in_rse_imm12_maybe_negated(
+                    ctx,
+                    inputs[1],
+                    ty_bits(ty),
+                    NarrowValueMode::None,
+                );
+                let alu_op = if !negated {
+                    choose_32_64(ty, ALUOp::Sub32, ALUOp::Sub64)
+                } else {
+                    choose_32_64(ty, ALUOp::Add32, ALUOp::Add64)
+                };
                 ctx.emit(alu_inst_imm12(alu_op, rd, rn, rm));
             } else {
                 let rm = put_input_in_reg(ctx, inputs[1], NarrowValueMode::None);