Use 'xor r, r' to set registers to 0 instead of mov (#766)

cranelift/codegen/meta/src/isa/x86/recipes.rs

@@ -1023,6 +1023,18 @@ pub(crate) fn define<'shared>(
             ),
     );
 
+    // XX+rd id unary with zero immediate.
+    recipes.add_template_recipe(
+        EncodingRecipeBuilder::new("u_id_z", f_unary_imm, 1)
+            .operands_out(vec![gpr])
+            .emit(
+                r#"
+                    {{PUT_OP}}(bits, rex2(out_reg0, out_reg0), sink);
+                    modrm_rr(out_reg0, out_reg0, sink);
+                "#,
+            ),
+    );
+
     // XX /n Unary with floating point 32-bit immediate equal to zero.
     {
         let format = formats.get(f_unary_ieee32);