[VPlan] Place VPExpandSCEVRecipe in pre-header.

After D121624 models the pre-header in VPlan, VPExpandSCEVRecipes can be
placed there. This ensures SCEV expansion happens before modifying the
CFG during VPlan execution, when CFG is incomplete.

Depends on D121624.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D122095