This optimizes bitwise AND with immediate mask values like 0xFFFF and 0xFFFFFFFF.
Currently, we do `andi` for these cases emitting 3 or 4 instructions repectively.
These are effectively zero extensions and could be reduced to 1 or 2 instructions
depending on whether Zba and Zbb extensions are available.

This also renames existing assembler routines zero/sign_extend to z/sext. This
will be compatible with the bit-manipulation spec which specifies names like sext.h
and zext.h. The various callsites look better to me after the renaming.