PostRAPseudos: Don't preserve kills on some implicit copy operands

This fixes a verifier error I ran into at -O0. A subregister copy had
an implicit kill of an overlapping superregister, which was partially
redefined by the copy. The preserved implicit operand killed
subregisters made live earlier in the sequence. AMDGPU already uses
similar logic for whether to preserve the kill of the superregister on
the final instruction if there's overlap.