[FIR] Mark more calls origin as OPERATOR

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

@@ -7,7 +7,9 @@ class A {
 
 fun main() {
     val a = A()
-    a[1]++
-    a[1] += 3
-    a[1] = a[1] + 3
+    <!OPERATOR_MODIFIER_REQUIRED!>a[1]<!>++
+    //fir prefers plusAssign call if neither get+set nor plusAssign resolverd
+    //hence UNRESOLVED_REFERENCE here
+    a[1] <!UNRESOLVED_REFERENCE!>+=<!> 3
+    <!OPERATOR_MODIFIER_REQUIRED!>a[1]<!> = a[1] + 3
 }