There are a lot of times when encoding AArch64 instructions that we
need to represent an integer value with a custom fixed width. For
example, the offset for a B instruction is 26 bits, so we store an
i32 on the instruction struct and then mask it when we encode.

We've been doing this masking everywhere, which has worked, but
it's getting a bit copy-pasty all over the place. This commit
centralizes that logic to make sure we stay consistent.