[llvm] Reland "[LoopVectorizer] Add support for chaining partial reductions #120272" (PR #124282)

[Nicholas Guy via llvm-commits]

================
@@ -142,9 +142,9 @@ class VPRecipeBuilder {
   /// Returns null if no scaled reduction was found, otherwise a pair with a
   /// struct containing reduction information and the scaling factor between the
   /// number of elements in the input and output.
----------------
NickGuy-Arm wrote:

Good catch, updated.

https://github.com/llvm/llvm-project/pull/124282