[FIR-TEST] Add new testdata generated after changes in previous commit

compiler/testData/diagnostics/tests/operatorsOverloading/kt11300.fir.kt

@@ -0,0 +1,13 @@
+// !WITH_NEW_INFERENCE
+
+class A {
+    operator fun get(x: Int): Int = x
+    fun set(x: Int, y: Int) {} // no `operator` modifier
+}
+
+fun main() {
+    val a = A()
+    a[1]++
+    a[1] += 3
+    a[1] = a[1] + 3
+}