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

compiler/testData/diagnostics/tests/regressions/AssignmentsUnderOperators.fir.kt

@@ -0,0 +1,6 @@
+fun test() {
+  var a : Any? = null
+  if (a is Any) else a = null;
+  while (a is Any) a = null
+  while (true) a = null
+}