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/inst/emit.rs

@@ -2138,13 +2138,6 @@ pub(crate) fn emit(
             sink.put1(*imm);
         }
 
-        Inst::XmmLoadConst { src, dst, ty } => {
-            let dst = allocs.next(dst.to_reg());
-            let load_offset = Amode::rip_relative(sink.get_label_for_constant(*src));
-            let load = Inst::load(*ty, load_offset, Writable::from_reg(dst), ExtKind::None);
-            load.emit(&[], sink, info, state);
-        }
-
         Inst::XmmUninitializedValue { .. } => {
             // This instruction format only exists to declare a register as a `def`; no code is
             // emitted.