Prior to this commit, the emitter would emit a 7-byte instruction when
loading a 32-bit immediate to a 64-bit register.
0: 48 c7 c0 ff ff ff 7f mov rax,0x7fffffff
With this change, it will check if it can instead emit a load to a
32-bit register, which takes only 5 or 6 bytes.
0: b8 ff ff ff 7f mov eax,0x7fffffff