[machinst x64]: rename Inst::XmmUninitializedValue and document

This approach is not the best but avoids an extra instruction; perhaps at some point, as mentioned in https://github.com/bytecodealliance/wasmtime/pull/2248, we will add the extra instruction or refactor things in such a way that this `Inst` variant is unnecessary.

cranelift/codegen/src/isa/x64/inst/emit.rs

@@ -2003,7 +2003,7 @@ pub(crate) fn emit(
             sink.bind_label(constant_end_label);
         }
 
-        Inst::XmmFakeDef { .. } => {
+        Inst::XmmUninitializedValue { .. } => {
             // This instruction format only exists to declare a register as a `def`; no code is
             // emitted.
         }