[LSR] Optimize unused IVs to final values in the exit block

Loop Strength Reduce sometimes optimizes away all uses of an induction variable
from a loop but leaves the IV increments. When the only remaining use of the IV
is the PHI in the exit block, this patch will call rewriteLoopExitValues to
replace the exit block PHI with the final value of the IV to skip the updates
in each loop iteration.

Differential Revision: https://reviews.llvm.org/D118808