[DAG]Introduce llvm::processShuffleMasks and use it for shuffles in DAG Type Legalizer.

We can process the long shuffles (working across several actual
vector registers) in the best way if we take the actual register
represantion into account. We can build more correct representation of
register shuffles, improve number of recognised buildvector sequences.
Also, same function can be used to improve the cost model for the
shuffles. in future patches.

Part of D100486

Differential Revision: https://reviews.llvm.org/D115653