[llvm] [VPlan] Introduce child regions as VPlan transform. (PR #129402)

[Florian Hahn via llvm-commits]

fhahn wrote:

> > I left the code dealing with adding operands to phis as-is (keep IR order for both predecessors and operands during intiial construction, canonicalize when region is introduced), pending adjustment if needed.
> 
> OK. Would it be better to apply this canonicalization when looking for region headers - swapping the predecessors of the header together with the operands of its phi recipes? Admittedly these predecessors are soon to be disconnected, but the number of headers (that require clear yet somewhat redundant swapping) is small.

Sounds good, updated, thanks!


https://github.com/llvm/llvm-project/pull/129402