ValueMapper: Add support for seeding metadata with nullptr

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.

llvm-svn: 265228