[llvm] [IR][LangRef] Add partial reduction add intrinsic (PR #94499)

[Paul Walker via llvm-commits]

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@@ -19209,6 +19209,37 @@ will be on any later loop iteration.
 This intrinsic will only return 0 if the input count is also 0. A non-zero input
 count will produce a non-zero result.
 
+'``llvm.experimental.vector.partial.reduce.add.*``' Intrinsic
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Syntax:
+"""""""
+This is an overloaded intrinsic.
+
+::
+
+      declare <4 x i32> @llvm.experimental.vector.partial.reduce.add.v4i32.v4i32.v8i32(<4 x i32> %accum, <8 x i32> %in)
+      declare <4 x i32> @llvm.experimental.vector.partial.reduce.add.v4i32.v4i32.v16i32(<4 x i32> %accum, <16 x i32> %in)
+      declare <vscale x 4 x i32> @llvm.experimental.vector.partial.reduce.add.nxv4i32.nxv4i32.nxv8i32(<vscale x 4 x i32> %accum, <vscale x 8 x i32> %in)
+      declare <vscale x 4 x i32> @llvm.experimental.vector.partial.reduce.add.nxv4i32.nxv4i32.nxv16i32(<vscale x 4 x i32> %accum, <vscale x 16 x i32> %in)
+
+Overview:
+"""""""""
+
+The '``llvm.vector.experimental.partial.reduce.add.*``' intrinsics perform an integer
+``ADD`` reduction of subvectors within a vector, before adding the resulting vector
+to the provided accumulator vector. The return type is a vector type that matches
+the type of the accumulator vector.
+
+Arguments:
+""""""""""
+
+The first argument is the accumulator vector, or a `zeroinitializer`. The type of
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paulwalker-arm wrote:

I don't think `zeroinitializer` adds anything to the description, as in, there's no change of behaviour base on this specific value.

https://github.com/llvm/llvm-project/pull/94499