Add a vscale test case

Author: spaits

llvm/test/Transforms/InstCombine/vector-reductions.ll

@@ -466,3 +466,16 @@ define i2 @constant_multiplied_7xi2(i2 %0) {
   %4 = tail call i2 @llvm.vector.reduce.add.v7i2(<7 x i2> %3)
   ret i2 %4
 }
+
+define i32 @negative_scalable_vector(i32 %0) {
+; CHECK-LABEL: @negative_scalable_vector(
+; CHECK-NEXT:    [[TMP2:%.*]] = insertelement <vscale x 4 x i32> poison, i32 [[TMP0:%.*]], i64 0
+; CHECK-NEXT:    [[TMP3:%.*]] = shufflevector <vscale x 4 x i32> [[TMP2]], <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
+; CHECK-NEXT:    [[TMP4:%.*]] = tail call i32 @llvm.vector.reduce.add.nxv4i32(<vscale x 4 x i32> [[TMP3]])
+; CHECK-NEXT:    ret i32 [[TMP4]]
+;
+  %2 = insertelement <vscale x 4 x i32> poison, i32 %0, i64 0
+  %3 = shufflevector <vscale x 4 x i32> %2, <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
+  %4 = tail call i32 @llvm.vector.reduce.add.nxv4i32(<vscale x 4 x i32> %3)
+  ret i32 %4
+}