Add an extension point for peephole optimizers.

This extension point allows adding passes that perform peephole optimizations
similar to the instruction combiner. These passes will be inserted after
each instance of the instruction combiner pass.

Differential Revision: http://reviews.llvm.org/D3905

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209595 91177308-0d34-0410-b5e6-96231b3b80d8