Support seeding a ValueMap with nullptr for Metadata entries, a
situation I didn't consider in the Metadata/Value split.
I added a ValueMapper::getMappedMD accessor that returns an
Optional<Metadata*> with the mapped (possibly null) metadata. IRMover
needs to use this to avoid modifying the map when it's checking for
unneeded subprograms. I updated a call from bugpoint since I find the
new code clearer.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265228 91177308-0d34-0410-b5e6-96231b3b80d8