[llvm] [VPlan] Introduce ExitPhi VPInstruction, use to create phi for FOR. (PR #94760)

[via llvm-commits]

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@@ -8522,6 +8476,58 @@ static void addUsersInExitBlock(VPBasicBlock *HeaderVPBB, Loop *OrigLoop,
   }
 }
 
+/// Feed a resume value for every FOR from the vector loop to the scalar loop,
+/// if middle block branches to scalar preheader, by introducing ExtractFromEnd
+/// and ResumePhi recipes in each, respectively, and a VPLiveOut which uses the
+/// latter and corresponds to the scalar header.
+static void addLiveOutsForFirstOrderRecurrences(VPlan &Plan) {
+  VPRegionBlock *VectorRegion = Plan.getVectorLoopRegion();
+
+  // Start by finding out if middle block branches to scalar preheader.
+  // TODO: Should be replaced by
+  // Plan->getScalarLoopRegion()->getSinglePredecessor() in the future once the
+  // scalar region is modeled as well.
+  VPBasicBlock *ScalarPHVPBB = nullptr;
+  auto *MiddleVPBB = cast<VPBasicBlock>(VectorRegion->getSingleSuccessor());
+  for (VPBlockBase *Succ : MiddleVPBB->getSuccessors()) {
+    if (isa<VPIRBasicBlock>(Succ))
+      continue;
+    assert(!ScalarPHVPBB && "Two candidates for ScalarPHVPBB?");
+    ScalarPHVPBB = cast<VPBasicBlock>(Succ);
+  }
+  if (!ScalarPHVPBB)
+    return;
+
+  for (auto &H : VectorRegion->getEntryBasicBlock()->phis()) {
+    auto *FOR = dyn_cast<VPFirstOrderRecurrencePHIRecipe>(&H);
+    if (!FOR)
+      continue;
+
+    VPBuilder B(ScalarPHVPBB);
----------------
ayalz wrote:

```suggestion
    VPBuilder ScalarPHBuilder(ScalarPHVPBB);
```


https://github.com/llvm/llvm-project/pull/94760