[PowerPC] fixupIsDeadOrKill start and end in different block fixing

In fixupIsDeadOrKill, we assume StartMI and EndMI not exist in same
basic block, so we add an assertion in that function. This is wrong
before RA, as before RA the true definition may exist in another
block through copy like instructions.

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D83365