In some cases folds done through ISel/DAGCombining result in SetCC
conditions that we can easily simplify using known bits.
This patch adds support for that.
NB: The medium-term goal is to get this patch in, then use comparisons
against zero for testing future improvements to isKnownNeverZero as
there are issues using cttz/ctlz for that.
Just curious: does "strengthening" conditions like this actually generate better code? Is it something we do elsewhere in the compiler?