Linker: Remove IRMover::isMetadataUnneeded indirection; almost NFC

Instead of checking live during MapMetadata whether a subprogram is
needed, seed the ValueMap with `nullptr` up-front.

There is a small hypothetical functionality change.  Previously, calling
MapMetadataOp on a node whose "scope:" chain led to an unneeded
subprogram would return nullptr.  However, if that were ever called,
then the subprogram would be needed; a situation that the IRMover is
supposed to avoid a priori!

Besides cleaning up the code a little, this restores a nice property:
MapMetadataOp returns the same as MapMetadata.

llvm-svn: 265229