[VPlan] Print successors of VPRegionBlocks.
The non-DOT printing does not include the successors of VPregionBlocks.
This patch use the same style for printing successors as for
I think the printing of successors could be a bit improved further, as
at the moment it is hard to ensure a check line matches all successors.
But that can be done as follow-up.
Reviewed By: a.elovikov
Differential Revision: https://reviews.llvm.org/D103515