[llvm] [InstCombine][NFC] Add test for existing fold (PR #181555)

[Tirthankar Mazumder via llvm-commits]

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@@ -3579,3 +3579,25 @@ define i1 @icmp_ult_add_lshr_neg_no_nuw(i32 %arg0) {
   %v2 = icmp ult i32 %v1, 256
   ret i1 %v2
 }
+
+define i1 @fold-icmp-sum-of-extended-i1(i16 %0, i16 %1, i16 %2, i16 %3) {
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wermos wrote:

I've renamed the variables and also added a little explanation for what the IR is doing.

https://github.com/llvm/llvm-project/pull/181555