[PATCH][X86] Add target specific combine rules to fold SSE/AVX/AVX2 blend intrinsics.

[Nadav Rotem]

LGTM.  Thanks Andrea. 

> On May 15, 2014, at 7:41, Andrea Di Biagio <andrea.dibiagio at gmail.com> wrote:
> 
> Hi,
> 
> This patch teaches the x86 backend how to fold SSE4.1/AVX/AVX2 blend
> intrinsics in the following trivial cases:
>  1)  fold (blend A, A, Mask) -> A;
>  2)  fold (blend A, B, <allZeros>) -> A;
>  3) fold (blend A, B, <allOnes>) -> B;
> 
> Added two new tests to verify that the new folding rules work for all
> the optimized blend intrinsics.
> 
> Please let me know if ok to submit.
> 
> Thanks,
> Andrea Di Biagio
> <patch-combine-x86-blend.diff>