[SCEV] See through op.with.overflow intrinsics

Summary:
This change teaches SCEV to see reduce `(extractvalue
0 (op.with.overflow X Y))` into `op X Y` (with a no-wrap tag if
possible).

Reviewers: atrick, regehr

Subscribers: mcrosier, mzolotukhin, llvm-commits

Differential Revision: http://reviews.llvm.org/D18684

llvm-svn: 265912