RFR (XS) 8210347 : Combine subsequent calls to Set.contains() and Set.add()

Hello! It's a pico-optimization. In a few places a call to Set.contains() can be merged with the following Set.add(). BUGURL: https://bugs.openjdk.java.net/browse/JDK-8210347 WEBREV: http://cr.openjdk.java.net/~igerasim/8210347/00/webrev/ Would you please help review the trivial fix? -- With kind regards, Ivan Gerasimov