[VPlan] Initial modeling of middle block in VPlan.

This patch extends the scope of VPlan to also include the exit (aka
middle) block.

For now, the exit block remains empty, but handling of exit values will
subsequently be moved to VPlan, by adding recipes to model exit values
in the exit block.

As a first step, this will allow fixing #51366.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D123457