There are some mistakes in code emission of x86 Assembler:
- testl(Address, int32_t) misses prefix(dst)
- addw(Register, Register) misses 0x66 prefix
- emit_operand(Register, Address) is often used incorrectly, maybe we should make the third argument explicit instead
- Assembler should not perform optimisations that change the actual instructions emitted, move these to MacroAssembler instead