[llvm] [VPlan] Model FOR resume value extraction in VPlan. (PR #93396)

[via llvm-commits]

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@@ -847,14 +847,84 @@ bool VPlanTransforms::adjustFixedOrderRecurrences(VPlan &Plan,
     // all users.
     RecurSplice->setOperand(0, FOR);
 
+    // This is the second phase of vectorizing first-order recurrences. An
+    // overview of the transformation is described below. Suppose we have the
+    // following loop.
+    //
+    //   for (int i = 0; i < n; ++i)
+    //     b[i] = a[i] - a[i - 1];
+    //
----------------
ayalz wrote:

```suggestion
    //
    // with some use after the loop of the last a[i-1], e.g.,
    //
    //   for (int i = 0; i < n; ++i) {
    //     t = a[i - 1];
    //     b[i] = a[i] - t;
    //   }
    //    use t;
    //
```

https://github.com/llvm/llvm-project/pull/93396