Remove xmm_r_r inst data structure and cases after related refactoring

Removes unneeded data structure that was holding instructions for
xmm based move instructions. These instructions can should be categorized
as rm not just r. This change is intended to simplify organization and
cases when lowering.

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

@@ -302,7 +302,7 @@ impl SseOpcode {
         }
     }
 
-    /// Returns src register operand size for an instruction
+    /// Returns the src operand size for an instruction
     pub(crate) fn src_size(&self) -> u8 {
         match self {
             SseOpcode::Movd => 4,