[llvm] [LV] Widen truncated induction updates, not just truncated phis (PR #220730)

[Ramkumar Ramachandra via llvm-commits]

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@@ -939,13 +954,41 @@ class LoopVectorizationCostModel {
 
   /// Return True if instruction \p I is an optimizable truncate whose operand
   /// is an induction variable. Such a truncate will be removed by adding a new
-  /// induction variable with the destination type.
-  bool isOptimizableIVTruncate(Instruction *I, ElementCount VF) {
+  /// induction variable with the destination type. On success, \p IVInfo, if
+  /// non-null, receives the matched phi and update.
+  bool isOptimizableIVTruncate(Instruction *I, ElementCount VF,
+                               OptimizableIVTruncateInfo *IVInfo = nullptr) {
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artagnon wrote:

I think this looks a lot like:

```cpp
/// Check if \p VPV is an untruncated wide induction, either before or after the
/// increment. If so return the header IV (before the increment), otherwise
/// return null.
static VPWidenInductionRecipe *
getOptimizableIVOf(VPValue *VPV, PredicatedScalarEvolution &PSE) {
```

I wonder if it's possible to lift the logic in question to VPlan? I think we want to avoid extending legacy code, and do as much as possible in VPlan?

https://github.com/llvm/llvm-project/pull/220730