[llvm] [VPlan] Narrow truncates of an induction plus a constant offset (PR #226035)

[Ramkumar Ramachandra via llvm-commits]

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@@ -929,7 +932,26 @@ getOptimizableIVOf(VPValue *VPV, PredicatedScalarEvolution &PSE) {
     }
     llvm_unreachable("should have been covered by switch above");
   };
-  return IsWideIVInc() ? WideIV : nullptr;
+  if (!PostIncStart)
+    return IsWideIVInc() ? WideIV : nullptr;
+
+  // start + C + i * step stays affine for any constant C, including the step,
+  // so it can be folded into the start value.
+  const APInt *C;
+  APInt Offset;
+  if (match(VPV, m_c_Add(m_Specific(WideIV), m_APInt(C))))
+    Offset = *C;
+  else if (match(VPV, m_Sub(m_Specific(WideIV), m_APInt(C))))
+    Offset = -*C;
+  else
+    return nullptr;
+
+  const APInt *StartC;
+  if (!match(WideIV->getStartValue(), m_APInt(StartC)))
+    return nullptr;
+  *PostIncStart =
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artagnon wrote:

```suggestion
  PostIncStart =
```

https://github.com/llvm/llvm-project/pull/226035