[LoopUnroll] Peel off iterations if it makes conditions true/false.

If the loop body contains conditions of the form IndVar < #constant, we
can remove the checks by peeling off #constant iterations.

This improves codegen for PR34364.

Reviewers: mkuper, mkazantsev, efriedma

Reviewed By: mkazantsev

Differential Revision: https://reviews.llvm.org/D43876

llvm-svn: 327671