[LV] Use VPValue to get expanded value for SCEV step expressions.

Update skeleton creation logic to use SCEV expansion results from
expanding the pre-header. This avoids another set of SCEV expansions
that may happen after the CFG has been modified.

Fixes #58811.

Depends on D147964.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D147965