[InstCombine] don't let 'exact' inhibit demanded bits folds for udiv

We shouldn't penalize instructions that have extra flags.

Drop the poison-generating flags if needed instead of bailing out.
This makes canonicalization/optimization more uniform.

There is a chance that dropping flags will cause some
other transform to not fire, but we added a preliminary
patch to avoid that with:
f0faea57

See D140665 for more details.