[InstCombine] Compute known bits for multi-use add/sub

We were failing to set the known bits for add/sub in the multi-use
case, resulting in odd behavioral differences depending on the
number of uses. Noticed while adding a consistency assertion.

The test changes are essentially a revert to the state before
d6498abc. These changes are not really desirable, but if we don't
want them, that needs to be handled as part of the heuristic for
demanded constant shrinking, not by artifically suppressing the
known bits in one specific case.