[llvm] [VPlan] Optimize FindLast of (binop %IV, live-in) by sinking. (PR #183911)

[via llvm-commits]

================
@@ -5783,16 +5823,34 @@ void VPlanTransforms::optimizeFindIVReductions(VPlan &Plan,
         BackedgeVal,
         match_fn(m_VPInstruction<VPInstruction::ComputeReductionResult>())));
 
+    // If IV is an expression, create a new select that tracks the underlying
+    // IV directly and update the backedge value.
+    if (ExpressionIV) {
+      auto *SelectR = CondSelect->getDefiningRecipe();
+      VPBuilder LoopBuilder(SelectR);
+      VPValue *SelectCond = Cond;
+      if (TrueVal == PhiR)
+        SelectCond = LoopBuilder.createNot(Cond);
+      CondSelect = LoopBuilder.createSelect(SelectCond, ExpressionIV, PhiR,
+                                            SelectR->getDebugLoc());
----------------
ayalz wrote:

Having both `CondSelect` and `SelectCond` may be confusing. Could `CondSelect` be better renamed, independently, `ReducingSelect` or `FindLastSelect` (it's only last we're after here, right?). Suggest to use `Cond` instead of `SelectCond` - retrieving it and TrueVal here than carry them over, can retain order of operands than reverse the condition:
```suggestion
      auto DL = SelectR->getDebugLoc();
      VPValue *Cond = SelectR->getOperand(0);
      if (SelectR->getOperand(1) == PhiR)
        CondSelect = LoopBuilder.createSelect(Cond, PhiR, ExpressionIV, DL);
      else
        CondSelect = LoopBuilder.createSelect(Cond, ExpressionIV, PhiR, DL);
```

https://github.com/llvm/llvm-project/pull/183911