cranelift: Add Bswap instruction (#1092) (#5147)

Adds Bswap to the Cranelift IR. Implements the Bswap instruction
in the x64 and aarch64 codegen backends. Cranelift users can now:
```
builder.ins().bswap(value)
```
to get a native byteswap instruction.

* x64: implements the 32- and 64-bit bswap instruction, following
the pattern set by similar unary instrutions (Neg and Not) - it
only operates on a dst register, but is parameterized with both
a src and dst which are expected to be the same register.

As x64 bswap instruction is only for 32- or 64-bit registers,
the 16-bit swap is implemented as a rotate left by 8.

Updated x64 RexFlags type to support emitting for single-operand
instructions like bswap

* aarch64: Bswap gets emitted as aarch64 rev16, rev32,
or rev64 instruction as appropriate.

* s390x: Bswap was already supported in backend, just had to add
a bit of plumbing

* For completeness, added bswap to the interpreter as well.

* added filetests and runtests for each ISA

* added bswap to fuzzgen, thanks to afonso360 for the code there

* 128-bit swaps are not yet implemented, that can be done later

cranelift/codegen/src/isa/x64/inst/mod.rs

@@ -68,6 +68,7 @@ impl Inst {
             Inst::AluRmiR { .. }
             | Inst::AluRM { .. }
             | Inst::AtomicRmwSeq { .. }
+            | Inst::Bswap { .. }
             | Inst::CallKnown { .. }
             | Inst::CallUnknown { .. }
             | Inst::CheckedDivOrRemSeq { .. }
@@ -1373,6 +1374,17 @@ impl PrettyPrint for Inst {
                 format!("{} {}", ljustify2("set".to_string(), cc.to_string()), dst)
             }
 
+            Inst::Bswap { size, src, dst } => {
+                let src = pretty_print_reg(src.to_reg(), size.to_bytes(), allocs);
+                let dst = pretty_print_reg(dst.to_reg().to_reg(), size.to_bytes(), allocs);
+                format!(
+                    "{} {}, {}",
+                    ljustify2("bswap".to_string(), suffix_bwlq(*size)),
+                    src,
+                    dst
+                )
+            }
+
             Inst::Cmove {
                 size,
                 cc,
@@ -1953,6 +1965,10 @@ fn x64_get_operands<F: Fn(VReg) -> VReg>(inst: &Inst, collector: &mut OperandCol
         Inst::Setcc { dst, .. } => {
             collector.reg_def(dst.to_writable_reg());
         }
+        Inst::Bswap { src, dst, .. } => {
+            collector.reg_use(src.to_reg());
+            collector.reg_reuse_def(dst.to_writable_reg(), 0);
+        }
         Inst::Cmove {
             consequent,
             alternative,