[X86][SSE] Add target shuffle support to X86TargetLowering::computeKnownBitsForTargetNode

Ideally we'd use resolveTargetShuffleInputs to handle faux shuffles as well but:
(a) that code path doesn't handle general/pre-legalized ops/types very well.
(b) I'm concerned about the compute time as they recurse to calls to computeKnownBits/ComputeNumSignBits which would need depth limiting somehow.

llvm-svn: 334007