x64: Ensure that constants are always 16 bytes for XmmMem (#4790)

Ensure that constants generated for the memory case of XmmMem values are always 16 bytes, ensuring that we don't accidantally perform an unaligned load.

Fixes #4761

cranelift/codegen/src/machinst/isle.rs

@@ -804,6 +804,12 @@ macro_rules! isle_prelude_methods {
             self.lower_ctx.use_constant(data)
         }
 
+        #[inline]
+        fn emit_u128_le_const(&mut self, value: u128) -> VCodeConstant {
+            let data = VCodeConstantData::Generated(value.to_le_bytes().as_slice().into());
+            self.lower_ctx.use_constant(data)
+        }
+
         #[inline]
         fn const_to_vconst(&mut self, constant: Constant) -> VCodeConstant {
             self.lower_ctx.use_constant(VCodeConstantData::Pool(