[SLPVectorizer] Tidyup isShuffle helper

Ensure we keep track of the input vectors in all cases instead of just for SK_Select.

Ideally we'd reuse the shuffle mask pattern matching in TargetTransformInfo::getInstructionThroughput here to easily add support for all TargetTransformInfo::ShuffleKind without mass code duplication, I've added a TODO for now but D48236 should help us here.

Differential Revision: https://reviews.llvm.org/D48023

llvm-svn: 334958