x64: port fcmp to ISLE (#3967)

* x64: port scalar `fcmp` to ISLE

Implement the CLIF lowering for the `fcmp` to ISLE. This adds a new
type-matcher, `ty_scalar_float`, for detecting uses of `F32` and `F64`.

* isle: rename `vec128` to `ty_vec12`

This refactoring changes the name of the `vec128` matcher function to
follow the `ty_*` convention of the other type matchers. It also makes
the helper an inline function call.

* x64: port vector `fcmp` to ISLE

cranelift/codegen/src/machinst/isle.rs

@@ -253,7 +253,16 @@ macro_rules! isle_prelude_methods {
             }
         }
 
-        fn vec128(&mut self, ty: Type) -> Option<Type> {
+        #[inline]
+        fn ty_scalar_float(&mut self, ty: Type) -> Option<Type> {
+            match ty {
+                F32 | F64 => Some(ty),
+                _ => None,
+            }
+        }
+
+        #[inline]
+        fn ty_vec128(&mut self, ty: Type) -> Option<Type> {
             if ty.is_vector() && ty.bits() == 128 {
                 Some(ty)
             } else {