[X86][AVX512] Fixed VPERMT2* shuffle mask decoding and enabled target shuffle combining.

Patch to add support for target shuffle combining of X86ISD::VPERMV3 nodes, including support for detecting unary shuffles.

This uncovered several issues with the X86ISD::VPERMV3 shuffle mask decoding of non-64 bit shuffle mask elements - the bit masking wasn't being correctly computed.

Removed non-constant pool mask decode path as we have no way of testing it right now.

Differential Revision: http://reviews.llvm.org/D17916

llvm-svn: 262809