[VPlan] Add vp_depth_first_shallow + graph traits for wrapper(NFC)

This patch adds a new VPBlockShallowTraversalWrapper struct to
provide graph traits specialization that do not traverse through
VPRegionBlocks. This matches the behavior of the existing traits for
plain VPBlockBase and is a step before moving the graph traits for
VPBlockBase to traverse through VPRegionBlocks to enable cross region
support in VPDominatorTree.

Depends on D140511.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D140512