[InstCombine] Move SSE/AVX vector blend folding to instcombiner

[InstCombine] Move SSE/AVX vector blend folding to instcombiner

As discussed in D11886, this patch moves the SSE/AVX vector blend folding to instcombiner from PerformINTRINSIC_WO_CHAINCombine (which allows us to remove this completely).

InstCombiner already had partial support for this, I just had to add support for zero (ConstantAggregateZero) masks and also the case where both selection inputs were the same (allowing us to ignore the mask).

I also moved all the relevant combine tests into InstCombine/blend_x86.ll

Differential Revision: http://reviews.llvm.org/D11934

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244723 91177308-0d34-0410-b5e6-96231b3b80d8