[CloneFunction] Don't crash if the value map doesn't hold something

It is possible for the value map to not have an entry for some value
that has already been removed.

I don't have a testcase, this is fall-out from a buildbot.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@277614 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed