[LoopUnroll] Simplify induction variables after peeling too.

Loop peeling also has an impact on the induction variables, so we should
benefit from induction variable simplification after peeling too.

Reviewers: sanjoy, bogner, mzolotukhin, efriedma

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D43878

llvm-svn: 328301