[MCRegInfo] Add forward sub and super register iterators. (NFC)

This patch adds forward iterators mc_difflist_iterator,
mc_subreg_iterator and mc_superreg_iterator, based on the existing
DiffListIterator. Those are used to provide iterator ranges over
sub- and super-register from TRI, which are slightly more convenient
than the existing MCSubRegIterator/MCSuperRegIterator. Unfortunately,
it duplicates a bit of functionality, but the new iterators are a bit
more convenient (and can be used with various existing iterator
utilities)  and should probably replace the old iterators in the future.

This patch updates some existing users.

Reviewers: evandro, qcolombet, paquette, MatzeB, arsenm

Reviewed By: qcolombet

Differential Revision: https://reviews.llvm.org/D70565