[VPlan] Use consecutive numbers to print VPValues instead of addresses.

Currently when printing VPValues we use the object address, which makes
it hard to distinguish VPValues as they usually are large numbers with
varying distance between them.

This patch adds a simple slot tracker, similar to the ModuleSlotTracker
used for IR values. In order to dump a VPValue or anything containing a
VPValue, a slot tracker for the enclosing VPlan needs to be created. The
existing VPlanPrinter can take care of that for the existing code. We
assign consecutive numbers to each VPValue we encounter in a reverse
post order traversal of the VPlan.

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

Reviewed By: gilr

Differential Revision: https://reviews.llvm.org/D73078