[InstCombine] Revisit user of newly one-use instructions

Many folds in InstCombine are limited to one-use instructions. For
that reason, if the use-count of an instruction drops to one, it
makes sense to revisit that one user. This is one of the most
common reasons why InstCombine fails to finish in a single iteration.

Doing this revisit actually slightly improves compile-time, because
we save an extra InstCombine iteration in enough cases to make a
visible difference.

This is conceptually NFC, but not NFC in practice, because differences
in worklist order can result in slightly different folding behavior.

The regressed tests in or-shifted-masks.ll now require a sequence of
instcombine,early-cse,instcombine to fold fully. D152876 would make
these fold in a single instcombine run again.

Differential Revision: https://reviews.llvm.org/D151807