[PATCH] [Reassociate] Better preserve NSW/NUW flags. (PR12985)

[Chad Rosier]

+ llvm-commits

Hi atrick, majnemer, chandlerc,

This change better preserves the NSW/NUW flags in the reassociation pass.

Part of PR12985.

http://reviews.llvm.org/D6172

Files:
  lib/Transforms/Scalar/Reassociate.cpp
  test/Transforms/Reassociate/wrap-flags.ll