[CodeGen] Fix peephole optimizer bug introduced in r205481. Fixes PR19318.

I should have read that comment a little more carefully. ;)

Regression test in the works, committing in the mean time to un-break people.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205511 91177308-0d34-0410-b5e6-96231b3b80d8