[VPlan] Disconnect VPRegionBlock from successors in graph iterator(NFCI)

This updates the VPAllSuccessorsIterator to not connect the
VPRegionBlock itself to its successors. The successors are connected to
the exit block of the region. At the moment, this doesn't change any
exisint functionality.

But the new schema ensures the following property when used for
VPDominatorTree:

1. Entry & exit blocks of regions dominate the successors of the region.

This allows for convenient checking of dominance between defs and uses
that are not defined in the same region. I will share a follow-up patch
to use it for the VPDominatorTree soon.

Depends on D140500.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D140511