InstCombine: Reimplementation of visitUDivOperand

This transform was originally added in r185257 but later removed in
r185415.  The original transform would create instructions speculatively
and then discard them if the speculation was proved incorrect.  This has
been replaced with a scheme that splits the transform into two parts:
preflight and fold.  While we preflight, we build up fold actions that
inform the folding stage on how to act.