[X86][AVX2] vpslldq/vpsrldq byte shifts for AVX2

This patch refactors the existing lowerVectorShuffleAsByteShift function to add support for 256-bit vectors on AVX2 targets.

It also fixes a tablegen issue that prevented the lowering of vpslldq/vpsrldq vec256 instructions.

Differential Revision: http://reviews.llvm.org/D7596

llvm-svn: 229311