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.isle

@@ -41,6 +41,16 @@
       (value_regs (imm $B64 1)
                   (imm $B64 0)))
 
+;;;; Rules for `f32const` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(rule (lower (f32const (u64_from_ieee32 x)))
+      (value_reg (imm $F32 x)))
+
+;;;; Rules for `f64const` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(rule (lower (f64const (u64_from_ieee64 x)))
+      (value_reg (imm $F64 x)))
+
 ;;;; Rules for `null` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (rule (lower (has_type ty (null)))