delete the MDNode that changed, rather than the other MDNode.
This is less work, because it doesn't require the changed node
to be re-inserted into the uniquing map and it doesn't require
the is-function-local flag to be recomputed. Also, it avoids
trouble when the existing node is part of a complicated
data structure.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114996 91177308-0d34-0410-b5e6-96231b3b80d8