LoopUnswitch - Simplify based on known not to a be constant.

Summary: In case we do not know what the condition is in an unswitched loop, but we know its definitely NOT a known constant. We can perform simplifcations based on this information.

Reviewers: sanjoy, hfinkel, chenli, efriedma

Reviewed By: efriedma

Subscribers: david2050, llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D28968

llvm-svn: 296041