[PATCH] DAGCombiner: Continue combining if FoldConstantArithmetic() fails.

[Ahmed Bougacha]

I think there are two parts here:

- checking the return value of FoldConstantArithmetic: that LGTM, though tests would of course be appreciated, if possible.
- disabling some optimizations on Opaque constants: that, I'll let Juergen comment on.  I'm opposed to it because of the brittleness, but pragmatically (and there's precedent) that's a reasonable solution.

For your assert failure, I'm fine with just fixing that for now (that would be r230355, correct?).

-Ahmed


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http://reviews.llvm.org/D6946

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