[LoopRotate] Rotate loops with loop exiting latches

If a loop has a loop exiting latch, it can be profitable
to rotate the loop if it leads to the simplification of
a phi node. Perform rotation in these cases even if loop
rotate itself didnt simplify the loop to get there.

Differential Revision: https://reviews.llvm.org/D44199

llvm-svn: 328933