[InstCombine] Fold add nsw + sadd.with.overflow

Fold `add nsw` and `sadd.with.overflow` with constants if the addition
does not overflow.

Part of https://bugs.llvm.org/show_bug.cgi?id=38146.

Patch by Dan Robertson.

Differential Revision: https://reviews.llvm.org/D58881

llvm-svn: 355530