[VPlan] Create initial skeleton before creating regions. (NFC)
Move out the logic to prepare for vectorization to a separate transform,
before creating loop regions. This was discussed as follow-up
in https://github.com/llvm/llvm-project/pull/136455.
This just moves the existing code around slightly and will simplify
follow-up patches to include the exiting edges during initial VPlan
construction.
llvm-project
d2ce88a9 - [VPlan] Create initial skeleton before creating regions. (NFC)
