[VPlan] Use recursive traversal iterator in VPSlotTracker.

This patch simplifies VPSlotTracker by using the recursive traversal
iterator to traverse all blocks in a VPlan in reverse post-order when
numbering VPValues in a plan.

This depends on a fix to RPOT (D100169). It also extends the traversal
unit tests to check RPOT.

Reviewed By: a.elovikov

Differential Revision: https://reviews.llvm.org/D100176