[X86][AVX] Support bit-blend integer shuffles for 256-bit integer vectors

AVX1 doesn't support the shuffling of 256-bit integer vectors. For 32/64-bit elements we get around this by shuffling as float/double but for 8/16-bit elements (assuming they can't widen) we currently just split, shuffle as 128-bit vectors and concatenate the results back.

This patch adds the ability to lower using the bit-blend patterns before defaulting to the splitting behaviour.

Part 2 of 2

Differential Revision: http://reviews.llvm.org/D17292

llvm-svn: 261082