When an MDNode changes to become identical to another MDNode,

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.

llvm-svn: 114996