Recommit "[VPlan] Unify Value2VPValue and VPExternalDefs maps (NFCI)."

This reverts the revert commit 8c2276f8.

The updated patch re-orders the getDefiningRecipe check in getVPalue to avoid
a use-after-free.

Original commit message:

    Before this patch, a VPlan contained 2 mappings for Values -> VPValue:
    1) Value2VPValue and 2) VPExternalDefs.

    This duplication is unnecessary and there are already cases where
    external defs are added to Value2VPValue. This patch replaces all uses
    of VPExternalDefs with Value2VPValue.

    It clarifies the naming of getOrAddVPValue (to getOrAddExternalVPValue)
    and addVPValue (to addExternalVPValue).

    At the moment, this is NFC, but will enable additional simplifications
    in D147783.

    Depends on D147891.

    Reviewed By: Ayal

    Differential Revision: https://reviews.llvm.org/D147892