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

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