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/args.rs

@@ -519,6 +519,12 @@ impl Into<SyntheticAmode> for Amode {
     }
 }
 
+impl Into<SyntheticAmode> for VCodeConstant {
+    fn into(self) -> SyntheticAmode {
+        SyntheticAmode::ConstantOffset(self)
+    }
+}
+
 impl PrettyPrint for SyntheticAmode {
     fn pretty_print(&self, _size: u8, allocs: &mut AllocationConsumer<'_>) -> String {
         match self {
@@ -527,7 +533,7 @@ impl PrettyPrint for SyntheticAmode {
             SyntheticAmode::NominalSPOffset { simm32 } => {
                 format!("rsp({} + virtual offset)", *simm32 as i32)
             }
-            SyntheticAmode::ConstantOffset(c) => format!("const({:?})", c),
+            SyntheticAmode::ConstantOffset(c) => format!("const({})", c.as_u32()),
         }
     }
 }