Merging r214385:

Merging r214385:
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r214385 | majnemer | 2014-07-30 21:49:29 -0700 (Wed, 30 Jul 2014) | 9 lines

InstCombine: Correctly propagate NSW/NUW for x-(-A) -> x+A

We can only propagate the nsw bits if both subtraction instructions are
marked with the appropriate bit.

N.B.  We only propagate the nsw bit in InstCombine because the nuw case
is already handled in InstSimplify.

This fixes PR20189.
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_35@223644 91177308-0d34-0410-b5e6-96231b3b80d8