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

[Paul Walker via llvm-commits]

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@@ -19209,6 +19209,38 @@ 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 reduce the
+input vector down to the number of elements dictated by the result vector, and
+then adds the resulting vector to the accumulator vector. The return type is a
+vector type that matches the type of the accumulator vector.
----------------
paulwalker-arm wrote:

Please re-read my previous review comments because this description is still too strict.  There is no accumulator vector (that's just an artefact of how LoopVectorize will use the intrinsic). There are just two vectors whose combined elements are to be reduced.

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