[VPlan] Add getPlan() to VPBlockBase.

This patch adds a getPlan accessor to VPBlockBase, which finds the entry
block of the plan containing the block and returns the plan set for this
block.

VPBlockBase contains a VPlan pointer, but it should only be set for
the entry block of a plan. This allows moving blocks without updating
the pointer for each moved block and in the future we might introduce a
parent relationship between plans and blocks, similar to the one in LLVM IR.

Reviewers: rengolin, hsaito, fhahn, Ayal, dorit, gilr

Reviewed By: gilr

Differential Revision: https://reviews.llvm.org/D74445