[SLP]Improve isGatherShuffledEntry by trying per-register shuffle.

Currently when building gather/buildvector node, we try to build nodes
shuffles without taking into account separate vector registers. We can
improve final codegen and the whole vectorization process by including
this info into the analysis and the vector code emission, allows to emit
better vectorized code.

Differential Revision: https://reviews.llvm.org/D149742