The result of GetArg() is returned by value, so this is essentially
still a copy. While the previous code *is* valid, this communicates what
is actually happening a little more explicitly.
This'll make it slightly nicer to do basic constant folding for 32-bit
and 64-bit variants of the same IR opcode type. By that, I mean it's
possible to inspect immediate values without a bunch of conditional
checks beforehand to verify that it's possible to call GetU32() or
GetU64, etc.
