[llvm] Add `llvm.vector.partial.reduce.fadd` intrinsic (PR #159776)

[Sander de Smalen via llvm-commits]

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@@ -20688,8 +20688,27 @@ Note that it has the following implications:
 -  If ``%cnt`` is non-zero, the return value is non-zero as well.
 -  If ``%cnt`` is less than or equal to ``%max_lanes``, the return value is equal to ``%cnt``.
 
+Vector Partial Reduction Intrinsics
+-----------------------------------
+
+Partial horizontal reductions of vectors can be expressed using the following intrinsics.
+Each one reduces the concatenation of the two vector arguments down to the number of elements
+of the result vector type.
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sdesmalen-arm wrote:

nit: odd formatting

https://github.com/llvm/llvm-project/pull/159776