[VPlan] Verify plan entry and exit blocks, set correct exit block.

Both the entry and exit blocks of the top-region of a plan must be
VPBasicBlocks. They also must have no predecessors or successors
respectively.

This invariant was broken when splitting a block for sink-after. To fix
the issue, set the exit block of the region *after* sink-after is done.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D114586