[FIR] Disallow operators not on functions

Introduce `NOT_FUNCTION_AS_OPERATOR` and use it instead of `PROPERTY_AS_OPERATOR`

^KT-65881 Fixed

Merge-request: KT-MR-14547

compiler/testData/diagnostics/tests/kt34857.fir.kt

@@ -0,0 +1,6 @@
+val Int.plusAssign: (Int) -> Unit
+    get() = {}
+
+fun main() {
+    1 <!NOT_FUNCTION_AS_OPERATOR!>+=<!> 2
+}