[SLP]Improve tryToGatherExtractElements by using per-register analysis.

Currently tryToGatherExtractElements function analyzes the whole vector,
regrdless number of actual registers, used in this vector. It may
prevent some optimizations, because per-register analysis may allow to
simplify the final code by reusing more already emitted vectors and
better shuffles.

Differential Revision: https://reviews.llvm.org/D148855