tighten up a couple of assertions. hitting the BitPosition == BitWidth case...

tighten up a couple of assertions. hitting the BitPosition == BitWidth case that was previously not caught resulted in nasty corruption of APInts that (on my system at least) could not be detected using UBSan, ASan, or Valgrind. this patch does not cause any extra failures in a check-all nor does it interfere with bootstrapping. David Blaikie informally approved this change.

llvm-svn: 347148