review for 6730716 (S): nulls from two unrelated classes compare not equal

[Chuck Rasbold]

http://webrev.invokedynamic.info/rasbold/6730716/

6730716: nulls from two unrelated classes compare not equal
Summary: check for not-nullness after proving that types are unrelated
Reviewed-by:

When the compiler finds that two objects have unrelated types, it
presumes that they must be not equal. Oops however, may be null, and
the compiler must check for non-nullness before presuming that two
oops compare not-equal.