[llvm] [VPlan] Update final IV exit value via VPlan. (PR #112147)

[via llvm-commits]

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@@ -666,6 +666,134 @@ static void legalizeAndOptimizeInductions(VPlan &Plan) {
   }
 }
 
+/// Return a wide IV, if \p VPV is an optimizable wide IV or wide IV use. That
+/// is, if \p VPV is either an untruncated wide induction, or if it increments a
+/// wide induction by its step.
+static VPWidenInductionRecipe *isOptimizableIVOrUse(VPValue *VPV) {
+  auto *WideIV = dyn_cast<VPWidenInductionRecipe>(VPV);
+  if (WideIV) {
+    // VPV itself is a wide induction, separately compute the end value for exit
+    // users if it is not a truncated IV.
+    if (isa<VPWidenPointerInductionRecipe>(WideIV) ||
+        !cast<VPWidenIntOrFpInductionRecipe>(WideIV)->getTruncInst())
+      return WideIV;
+    return nullptr;
+  }
+
+  // Check if VPV is an optimizable induction increment.
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ayalz wrote:

Hmm, this is indeed confusing; should `fixHeaderPhis()` also hookup the operand across the backedge of induction header phi recipes, as it does for reductions and FORs? Thereby supporting its `VPHeaderPHIRecipe::getBackedgeValue()` API. Admittedly, this may contradict #82270(?)

https://github.com/llvm/llvm-project/pull/112147