[X86][SSE] Add support for target shuffle constant folding

Initial support for target shuffle constant folding in cases where all shuffle inputs are constant. We may be able to relax this and merge shuffles with only some constant inputs in the future.

I've added the helper function getTargetConstantBitsFromNode (based off a similar function in X86ShuffleDecodeConstantPool.cpp) that could be reused for other cases requiring constant vector extraction.

Differential Revision: https://reviews.llvm.org/D27220

llvm-svn: 288250