Constant fold switch inst when looking for trivial conditions to unswitch on.

Summary: Constant fold switch inst when looking for trivial conditions to unswitch on.

Reviewers: sanjoy, chenli, hfinkel, efriedma

Subscribers: llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D29037

llvm-svn: 293250