[PowerPC] Fix broken kill flag after MI peephole

The test case in https://bugs.llvm.org/show_bug.cgi?id=47373 exposed
two bugs in the PPC back end. The first one was fixed in commit
27714075 but the test case had to
be added without -verify-machineinstrs due to the second bug.
This commit fixes the use-after-kill that is left behind by the
PPC MI peephole optimization.