Prior to this commit, the emitter would unconditionally emit a 10-byte
instruction known as MOVABS when loading a 64-bit immediate to a
register.
0: 48 b8 ef be ad de 00 movabs rax,0xdeadbeef
7: 00 00 00
With this change, it will instead rely on the fact that on x64 writes to
32-bit registers are automatically zero extended to 64-bits, allowing
us to emit a 5 or 6-bytes instruction with the same effect for certain
immediates.
0: b8 ef be ad de mov eax,0xdeadbeef
53a947749a