[InstCombine] Fold abs of known negative operand

If we know that the abs operand is known negative, we can replace
it with a neg.

To avoid computing known bits twice, I've removed the fold for the
non-negative case from InstSimplify. Both the non-negative and the
negative case are handled by InstCombine now, with one known bits call.

Differential Revision: https://reviews.llvm.org/D87196