[VPlan] Move initial skeleton construction to createInitialVPlan. (NFC)

This patch moves creating the  middle VPBBs and an initial empty
vector loop region for the top-level loop to createInitialVPlan.

This consolidates code to create the initial VPlan skeleton and enables
adding other bits outside the main region during initial VPlan
construction. In particular, D150398 will add the exit check & branch to
the middle block.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D158333