added sum to skipped case for cse

refs ticket:4850

Belonging to [master]:
  - OpenModelica/OMCompiler#2332
  - OpenModelica/OpenModelica-testsuite#910

Author: vruge

Compiler/BackEnd/CommonSubExpression.mo

@@ -1278,6 +1278,7 @@ algorithm
     case DAE.CALL(path=Absyn.IDENT("sqrt")) then true;
     case DAE.CALL(path=Absyn.IDENT("String")) then true;
     case DAE.CALL(path=Absyn.IDENT("subSample")) then true;
+    case DAE.CALL(path=Absyn.IDENT("sum")) then true;
     case DAE.CALL(path=Absyn.IDENT("superSample")) then true;
     case DAE.CALL(path=Absyn.IDENT("terminal")) then true;
     case DAE.CALL() guard(Expression.isImpureCall(inCall) or isCallRecordConstructor(inCall, functionTree)) then true;