[InstCombine][KnownBits] Use KnownBits better to detect nsw adds

Change checkRippleForAdd from a heuristic to a full check -
if it is provable that the add does not overflow return true, otherwise false.

Patch by Yoav Ben-Shalom

Differential Revision: https://reviews.llvm.org/D32686

llvm-svn: 302093