[VPlan] Add dump function to VPlan class.

This adds a dump() function to VPlan, which uses the existing
operator<<.

This method provides a convenient way to dump a VPlan while debugging,
e.g. from lldb.

Reviewers: hsaito, Ayal, gilr, rengolin

Reviewed By: hsaito

Differential Revision: https://reviews.llvm.org/D70920