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

[via llvm-commits]

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@@ -648,6 +648,22 @@ bool VPBasicBlock::isExiting() const {
   return getParent() && getParent()->getExitingBasicBlock() == this;
 }
 
+std::optional<std::pair<VPBasicBlock *, VPBasicBlock *>>
+VPBasicBlock::isHeader(const VPDominatorTree &VPDT) const {
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ayalz wrote:

Better retain as a static isHeader(VPBB, VPDT) rather than a method of VPBB which depends on an external analysis (or a VPDT::isHeader(VPBB) method)?

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