Micro-optimization: Shorter encoding of MOVZX in more cases on x86-64

Micro-optimization: Shorter encoding of MOVZX in more cases on x86-64

The assembler instruction MOVZX is already assembled as a straight
32-bit MOV when it is used to zero-extend a 32-bit source into a 64-bit
register, taking advantage of the processor's implicit zero extension,
sparing the REX prefix (if it was only needed to indicate the 64-bit
size) and thus reducing code size. Towards the same goal, change
zero-extensions of 8- or 16-bit sources into a 64-bit register to use
the corresponding 32-bit register as the destination instead, in the
process refactoring EMIT-MOVE-WITH-EXTENSION for more OAOO-ness.