[InstCombine] try multi-use demanded bits folds for 'add'

This patch enables a multi-use demanded bits fold (motivated by issue #57576):
https://alive2.llvm.org/ce/z/DsZakh

This mimics transforms that we already do on the single-use path.

Originally, this patch did not include the last part to form a constant, but
that can be removed independently to reduce risk. It's not clear what the
effect of either change will be when viewed end-to-end.

This is expected to be neutral or a slight win for compile-time.
See the "add-demand2" series for experimental timing results:
https://llvm-compile-time-tracker.com/?config=NewPM-O3&stat=instructions&remote=rotateright

Differential Revision: https://reviews.llvm.org/D133788