[VPlan] Track current vector loop in VPTransformState (NFC).

Instead of looking up the vector loop using the header, keep track of
the current vector loop in VPTransformState. This removes the
requirement for the vector header block being part of the loop up front.

A follow-up patch will move the code to generate the Loop object for the
vector loop to VPRegionBlock.

Depends on D121619.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D121621