RFR: 8204322: "+=" applied to String operands can provoke side effects

[Vicente Romero]

Please review the fix for [1] at [2]. Javac was generating incorrect 
code for the case when the `+=` operand was applied to Strings and it is 
visible for test cases like:

public class Test {
    static int test() {
        System.out.println("evaluated");
        return 0;
    }

    public static void main(String[] args) {
        String[] array = {""};
        array[test()] += "a";
    }
}

the string "evaluated" is printed twice. Some items were missing in the 
stack, the solution is to duplicate those missing items plus not to 
evaluate the lhs expression twice.

Thanks in advance for the comments,
Vicente

Thanks to Aleksey Shipilev for the great regression test.

[1] https://bugs.openjdk.java.net/browse/JDK-8204322
[2] http://cr.openjdk.java.net/~vromero/8204322/webrev.01/

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