[llvm] [VPlan] Hook IR blocks into VPlan during skeleton creation (NFC) (PR #114292)

[via llvm-commits]

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@@ -1276,6 +1287,19 @@ VPlan *VPlan::duplicate() {
   return NewPlan;
 }
 
+VPBasicBlock *VPlan::getScalarPreheader() {
+  auto *MiddleVPBB =
+      cast<VPBasicBlock>(getVectorLoopRegion()->getSingleSuccessor());
+  if (MiddleVPBB->getNumSuccessors() == 2) {
+    // Order is strict: first is the exit block, second is the scalar preheader.
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ayalz wrote:

Simpler to return last successor, provided it ain't a VPIRBB?

https://github.com/llvm/llvm-project/pull/114292