[LICM] Extend the MustExecute scope

CanProveNotTakenFirstIteration utility does not handle the case when
condition of the branch is a constant. Add its handling.

Reviewers: reames, anna, mkazantsev
Reviewed By: reames
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D46996

llvm-svn: 332695