[LV] Remove unneeded createHeaderBranch.(NFCI)

The only remaining use was to get the exit block of the loop. Instead of
relying on the loop, use the successor of VectorHeaderBB
(LoopMiddleBlock) directly to set VPTransformState::CFG::ExitB

Depends on D121621.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D121623