[LV] Use VPValue for SCEV expansion in fixupIVUsers.

The step is already expanded in the VPlan. Use this expansion instead.
This is a step towards modeling fixing up IV users in VPlan.

 It also fixes a crash casued by SCEV-expanding the Step expression in
fixupIVUsers, where the IR is in an incomplete state

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D147963