[LV] Reuse SCEV expansion results for epilogue vectorization.

When generating code for the epilogue vector loop, we need to re-use the
expansion results for induction steps generated for the main vector
loop, as the pre-header of the epilogue vector loop may not dominate the
vector preheader of the epilogue.

This fixes a reported crash. Note that this is a workaround which should
be removed soon once induction resume value creation is handled in VPlan
directly.