After #62560, if a loop pre-header is created before loop cloning (say, immediately after, or during, loop recognition), loop cloning breaks the pre-header invariants by inserting loop choice conditions after the pre-header and before the cloned loops.

Fix this so loop cloning preserves the pre-header, if it exists, so the pre-header invariants (and assertion checking) works.

category:cq
theme:loop-opt
skill-level:expert
cost:medium
impact:small