Utils/LoopUnroll.cpp: Tweak (StringRef)OldName to be valid until it is used, since r194601.

eraseFromParent() invalidates OldName.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194970 91177308-0d34-0410-b5e6-96231b3b80d8