x64: remove Inst::XmmLoadConst (#4876)

This is a cherry-pick of a long-ago commit, 2d46637. The original
message reads:

> Now that `SyntheticAmode` can refer to constants, there is no longer a
> need for a separate instruction format--standard load instructions will
> work.

Since then, the transition to ISLE and the use of `XmmLoadConst` in many
more places makes this change a larger diff than the original. The basic
idea is the same, though: the extra indirection of `Inst::XMmLoadConst`
is removed and replaced by a direct use of `VCodeConstant` as a
`SyntheticAmode`. This has no effect on codegen, but the CLIF output is
now clearer in that the actual instruction is displayed (e.g., `movdqu`)
instead of a made-up instruction (`load_const`).

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

@@ -412,6 +412,11 @@ impl Context for IsleContext<'_, '_, MInst, Flags, IsaFlags, 6> {
         amode.clone().into()
     }
 
+    #[inline]
+    fn const_to_synthetic_amode(&mut self, c: VCodeConstant) -> SyntheticAmode {
+        SyntheticAmode::ConstantOffset(c)
+    }
+
     #[inline]
     fn writable_gpr_to_reg(&mut self, r: WritableGpr) -> WritableReg {
         r.to_writable_reg()