[llvm] [SLP] Support ordered fadd reduction via reduction intrinsics (PR #189451)

[Ryan Buchner via llvm-commits]

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@@ -29046,6 +29138,239 @@ class HorizontalReduction {
     return VectorizedTree;
   }
 
+  /// Attempt to vectorize an ordered (linearized) reduction chain.
+  /// Reduced values from matchOrderedReduction() are in accumulation order.
+  /// Vectorized subsets are immediately reduced via ordered reduction
+  /// intrinsics; non-vectorized values are folded linearly.
+  Value *tryToReduceOrdered(BoUpSLP &V, const DataLayout &DL,
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bababuck wrote:

Why use `V` for `BoUpSLP`, isn't `R` normally used in the codebase?
```suggestion
  Value *tryToReduceOrdered(BoUpSLP &R, const DataLayout &DL,
```

https://github.com/llvm/llvm-project/pull/189451