[PATCH] Add an extension point for peephole optimizers.

[Chandler Carruth]

This makes a lot of sense to me, and the patch LGTM. Please commit.

I wonder if we want to distinguish between an early, canonicalizing peephole and a late target-specific peephole? Certainly, this patch is fine, just food for thought if this ever comes up.

http://reviews.llvm.org/D3905