LibJIT: Widen allowed argument range for add32 and use REX if necessary

The REX prefix is elided when it is not needed, so no change in code
size is to be expected

Userland/Libraries/LibJIT/Assembler.h

@@ -615,14 +615,17 @@ struct Assembler {
 
     void add32(Operand dst, Operand src, Optional<Label&> label)
     {
-        if (dst.type == Operand::Type::Reg && to_underlying(dst.reg) < 8 && src.type == Operand::Type::Reg && to_underlying(src.reg) < 8) {
+        if (dst.is_register_or_memory() && src.type == Operand::Type::Reg) {
+            emit_rex_for_mr(dst, src, REX_W::No);
             emit8(0x01);
             emit_modrm_mr(dst, src);
-        } else if (dst.type == Operand::Type::Reg && to_underlying(dst.reg) < 8 && src.type == Operand::Type::Imm && src.fits_in_i8()) {
+        } else if (dst.is_register_or_memory() && src.type == Operand::Type::Imm && src.fits_in_i8()) {
+            emit_rex_for_slash(dst, REX_W::No);
             emit8(0x83);
             emit_modrm_slash(0, dst);
             emit8(src.offset_or_immediate);
-        } else if (dst.type == Operand::Type::Reg && to_underlying(dst.reg) < 8 && src.type == Operand::Type::Imm && src.fits_in_i32()) {
+        } else if (dst.is_register_or_memory() && src.type == Operand::Type::Imm && src.fits_in_i32()) {
+            emit_rex_for_slash(dst, REX_W::No);
             emit8(0x81);
             emit_modrm_slash(0, dst);
             emit32(src.offset_or_immediate);