[AArch64][CodeGen] Fix illegal register aliasing bug for mops instrs (#88869)

A bug was found where mops instructions were being generated that
aliased the source and size registers. This is unpredictable behaviour.
This patch uses the earlyclobber constraint on the input source
register so that it doesn't alias with the size register. Also a test is
introduced which checks affected instructions can't violate this
constraint.