Fix a problem exposed by my previous commit and noticed by a release-asserts

Fix a problem exposed by my previous commit and noticed by a release-asserts
buildbot: the debugging and non-debugging versions of getFunction were not
functionally equivalent: the non-debugging version wrongly assumed that if a
metadata operand was not metadata, then it had a non-null containing function.
This is not true, since the operand might be a global value, constant etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103008 91177308-0d34-0410-b5e6-96231b3b80d8