LibJS: Don't make Bytecode::Operand::Type an u8 outside of aarch64

This is a slightly mystified attempt to recover the performance
regression seen on our JS benchmark runner after 3c2a2bb39f
and c7bba505ea.

With this change, c7bba505ea is effectively reverted from the
perspective of x86_64.

Libraries/LibJS/Bytecode/Operand.h

@@ -13,7 +13,11 @@ namespace JS::Bytecode {
 
 class Operand {
 public:
-    enum class Type : u8 {
+    enum class Type
+#if ARCH(AARCH64)
+        : u8
+#endif
+    {
         Invalid,
         Register,
         Local,