[llvm] [VPlan] Optimize FindLast of FindIV w/o sentinel. (PR #172569)

[Mel Chen via llvm-commits]

================
@@ -5526,36 +5526,83 @@ void VPlanTransforms::optimizeFindIVReductions(VPlan &Plan,
         CheckSentinel(IVSCEV, UseMax, /*IsSigned=*/true);
     if (!SentinelVal) {
       SentinelVal = CheckSentinel(IVSCEV, UseMax, /*IsSigned=*/false);
-      if (!SentinelVal)
-        continue;
       UseSigned = false;
     }
 
-    auto *RdxResult = cast<VPInstruction>(vputils::findRecipe(
+    // Fall back to boolean tracking if IV doesn't wrap.
+    if (!SentinelVal) {
+      auto *AR = cast<SCEVAddRecExpr>(IVSCEV);
+      if (AR->hasNoSignedWrap()) {
+        UseSigned = true;
+      } else if (AR->hasNoUnsignedWrap()) {
+        UseSigned = false;
+      } else {
+        continue;
+      }
----------------
Mel-Chen wrote:

```suggestion
      if (AR->hasNoSignedWrap())
        UseSigned = true;
      else if (AR->hasNoUnsignedWrap()) 
        UseSigned = false;
      else 
        continue;
```

https://github.com/llvm/llvm-project/pull/172569