[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
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paulwalker-arm wrote:

This should be loosed to unrestricted the way the operands are combined.  In its broadest sense the operands are concatenated into a single vector that's then reduced down to the number of elements dictated but the result type (and hence first operand type) but there's no specification for how the reduction is distributed throughout those elements.

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