The memory operand might use one of the extended GPRs R8 through R15 and
hence require a REX prefix, but having a REX prefix makes the high-byte
register AH unencodeable as the src operand. This latent bug was exposed
by this branch, presumably because `select` now happens to be putting
something in an extended GPR instead of a legacy GPR.
In theory this could be fixed with minimal cost by introducing a way to
communicate to `select` that neither the destination memory nor the
other temporary can be in an extended GPR. However, I just went for the
simple solution which comes at a cost of one trivial instruction: copy
the remainder from AH to AL, and *then* copy AL to the destination.
`rep movsb` isn't usually a great idea here. This commit makes the logic
which tentatively existed in `genInlineMemcpy` apply in more cases, and
in particular applies it to the "new" backend logic. Put simply, all
copies of 128 bytes or fewer will now attempt this path first,
where---provided there is an SSE register and/or a general-purpose
register available---we will lower the operation using a sequence of 32,
16, 8, 4, 2, and 1 byte copy operations.
The feedback I got on this diff was "Push it to master and if it
miscomps I'll revert it" so don't blame me when it explodes
