[PATCH] Disable DAGCombine for -O0 and optnone

[Oleg Ranevskyy]

In http://reviews.llvm.org/D9992#182775, @mkuper wrote:

> I'm not sure it's a good idea to move the entire combine into lowering.
>  The FNEG treatment is an actual combine. And I'm not 100% it will work correctly now (e.g. in cases where there are multiple users for the FNEG).
>
> It would probably be better to leave the combine as is, but add a trivial "safety-net" lowering for ISD::FMA -> X86ISD::FMADD.


Thanks Michael. I will look into this.


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http://reviews.llvm.org/D9992

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