[PATCH] [InstCombine] Don't propagate nsw for A*B+A*C => A*(B+C).

[Sanjoy Das]

Hi majnemer, IlyaKrotov,

InstCombine transform `A *nsw B +nsw A *nsw C` to `A *nsw (B + C)`.
This is incorrect -- e.g. if A = -1, B = 1, C = INT_SMAX.  Then
nothing in the LHS overflows, but the multiplication in RHS overflows.

I'll have to admit that this proposed fix is a bit heavy-handed -- if
you think this will materially affect optimization then we can try to do
this transform for cases where we can prove correctness.  For instance,
I *think* (but have not proved) that we can transform `(A +nsw A) +nsw
A` to `A *nsw 3`.

http://reviews.llvm.org/D9629

Files:
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/add2.ll

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