[SLP]Integrate looking through shuffles logic into ShuffleInstructionBuilder.

Added BaseShuffleAnalysis as a base class for ShuffleInstructionBuilder
and integrated shuffle logic from shuffles for externally used scalars
into this class. This class is used as the main container that
implements smart shuffle instruction builder logic.
ShuffleInstructionBuilder uses this logic.
ShuffleInstructionBuilder is also used in building of the shuffle for
the externally used scalars instead of lambdas, which are now part of BaseShuffleAnalysis class.

Differential Revision: https://reviews.llvm.org/D140100