isle: Migrate f32const/f64const to ISLE (#3537)

This moves the `f32const` and `f64const` instructions from `lower.rs` to
ISLE. I was originally going to add something else but due to the
`isle.rs`'s manual use of `constructor_imm(..)` it necessitated filling
out the `imm` cases for f32/f64 constants, so I figured I'd go ahead and
move these instructions as well.

The special case for 0 constants which use `xorp{s,d}` is preserved from
`lower.rs` today, but a special case isn't added for the all-ones
constant. The comment says to use `cmpeqp{s,d}` but as discovered on
other recent PRs that's not quite sufficient because comparing a
register against itself which happens to be NaN wouldn't work, so
something else will be required (perhaps `pcmpeq` or similar? I figured
I'd defer to later)

cranelift/codegen/src/isa/x64/lower.rs

@@ -3361,25 +3361,19 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
         }
 
         Opcode::F64const => {
-            // TODO use cmpeqpd for all 1s.
-            let value = ctx.get_constant(insn).unwrap();
-            let dst = get_output_reg(ctx, outputs[0]);
-            for inst in Inst::gen_constant(dst, value as u128, types::F64, |ty| {
-                ctx.alloc_tmp(ty).only_reg().unwrap()
-            }) {
-                ctx.emit(inst);
-            }
+            unreachable!(
+                "implemented in ISLE: inst = `{}`, type = `{:?}`",
+                ctx.dfg().display_inst(insn),
+                ty
+            );
         }
 
         Opcode::F32const => {
-            // TODO use cmpeqps for all 1s.
-            let value = ctx.get_constant(insn).unwrap();
-            let dst = get_output_reg(ctx, outputs[0]);
-            for inst in Inst::gen_constant(dst, value as u128, types::F32, |ty| {
-                ctx.alloc_tmp(ty).only_reg().unwrap()
-            }) {
-                ctx.emit(inst);
-            }
+            unreachable!(
+                "implemented in ISLE: inst = `{}`, type = `{:?}`",
+                ctx.dfg().display_inst(insn),
+                ty
+            );
         }
 
         Opcode::WideningPairwiseDotProductS => {