JVM_IR don't use Intrinsics.stringPlus for 2-argument concatenation

compiler/testData/codegen/box/strings/twoArgumentNullableStringOperatorPlus.kt

@@ -0,0 +1,17 @@
+fun stringPlus(x: String?, y: Any?) = x + y
+
+fun box(): String {
+    val t1 = stringPlus("a", "b")
+    if (t1 != "ab") return "Failed: t1=$t1"
+
+    val t2 = stringPlus("a", null)
+    if (t2 != "anull") return "Failed: t2=$t2"
+
+    val t3 = stringPlus(null, "b")
+    if (t3 != "nullb") return "Failed: t3=$t3"
+
+    val t4 = stringPlus(null, null)
+    if (t4 != "nullnull") return "Failed: t4=$t4"
+
+    return "OK"
+}