Recommit "[LV] Remove unneeded createHeaderBranch.(NFCI)"

This reverts the revert commit 2760cdc9.

This version pulls in the code to create the vector loop object in VPlan
from D121624.

This is needed because otherwise existing LoopInfo verification will
fail, as a loop block doesn't have in-loop successors now that we
do not replace the branch.

Now that we do not add new loops during skeleton construction, there's
also no need to verify LI there.