Remember to clear the shadow kill flag at the same time as clearing the real

Remember to clear the shadow kill flag at the same time as clearing the real
kill flag.

This could cause duplicate kill flags when the same register was used twice in a
continuous sequence of STRs.

There is no small test case. <rdar://problem/8218046>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112534 91177308-0d34-0410-b5e6-96231b3b80d8