[IndVars] Break backedge and replace PHIs if loop exits on 1st iteration

Implement TODO in optimizeLoopExits. Now if we have proved that some loop exit
is taken on 1st iteration, we make all branches in the following exiting blocks
always branch out of the loop and their conditions simplified away.

Patch by Dmitry Makogon!

Differential Revision: https://reviews.llvm.org/D108910
Reviewed By: lebedev.ri