[VPlan] Switch default graph traits to be recursive, update VPDomTree.

This updates the GraphTraits specialization for VPBlockBase to recurse
through VPRegionBlocks.

This in turn enables using VPDominatorTree to query dominance between
any block in a plan. This should enable additional use cases, including
improvements to def-use verification and porting IR-based transforms
that rely on the dominator tree.

Specifically, this change means that for regions, the entry and exit
blocks dominate the successors of the region.

Depends on D140512 and D142162.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D140513