[LV] Move all VPlan transforms after initial VPlan construction.

Reorder VPlan transforms slightly so they are all grouped together,
after disabling Value -> VPValue lookup. In terms of codegen impact,
this should be NFC modulo a small number of instruction reorderings.

Preparation to split up tryToBuildVPlanWithVPRecipes in a follow-up.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D154640