[SLP][NFC]Initial redesign of ShuffleInstructionBuilder, NFC.

The patch redesigns ShuffleInstructionBuilder so it could later be used
for reshuffling of the buildvector sequences and vectorized parts of
  externally used scalars. Also will allow to generalize cost model for
  the gathers/buildvectors.

Part of D110978.

Differential Revision: https://reviews.llvm.org/D139718