[SCEV] Refactor isHighCostExpansionHelper

To enable the cost of constants, the helper function has been
reorganised:
- A struct has been introduced to hold SCEV operand information so
  that we know the user of the operand, as well as the operand index.
  The Worklist now uses instead instead of a bare SCEV.
- The costing of each SCEV, and collection of its operands, is now
  performed in a helper function.

Differential Revision: https://reviews.llvm.org/D86050