[PATCH] generate extract_subvector node to avoid disastrous shuffle vector codegen

[Michael Kuperstein]

I looked at PR15872 yesterday, and ended up with something very similar, although more general. 
Too bad I didn't really look at this review before starting. :-\

Then again, maybe I ended up too general, although I don't think I'm running into the miscompile.
Will add you to the review for that in a few minutes, but I'm fine with either version, or some merge of them, as long as it solves both PRs (and I'd still like someone more experienced with this to give a LGTM :-) ).


http://reviews.llvm.org/D6622

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