[LoopUnroll] Simplify loops created by unrolling.

Summary:
Currently loop-unrolling doesn't preserve loop-simplified form. This patch
fixes it by resimplifying affected loops.

Reviewers: chandlerc, sanjoy, hfinkel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23148

llvm-svn: 278038