[SLP][NFC] Outline lookahead heuristics into a separate helper class.

Minor refactoring to reduce size of functional change D124309:
  look-ahead scoring routines pulled out of VLOperands and formed
  new LookAheadHeuristics helper class.

Reviewed By: Alexey Bataev (ABataev), Vasileios Porpodas (vporpo)
Differential Revision: https://reviews.llvm.org/D124313