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RELEASE_500/final

RELEASE_500/final: Merging r312285:
@zmodem zmodem tagged this 01 Sep 16:28 
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r312285 | ctopper | 2017-08-31 14:39:23 -0700 (Thu, 31 Aug 2017) | 11 lines

[X86] Don't pull carry through X86ISD::ADD carryin, -1 if we can't guranteed we're really using the carry flag from the add.

Prior to this patch we had a DAG combine that tried to bypass an X86ISD::ADD with -1 being added to the carry flag of some previous operation. We would then pass the carry flag directly to user.

But this is only safe if the user is looking for the carry flag and not the zero flag.

So we need to only do this combine in a context where we know what flag the consumer is using.

Fixes PR34381.

Differential Revision: https://reviews.llvm.org/D37317
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