RFR: 8155043: BitMap set operations assume clear bits beyond unaligned end

[Kim Barrett]

Please review this change to the BitMap set operations to avoid
depending on or modifying bits beyond the size of the BitMap when the
size is not word aligned.  This change also adds some unit tests for
these operations.

In addition:

- Changed set_from to use Copy::disjoint_words.  For other set
operations, use a consistent iteration idiom for all.

- In the set_xxx_with_result operations, eliminated conditionals in
the accumulation of the result.

- Removed unused BitMap::set_intersection_at_offset, rather than
writing tests for it.

CR:
https://bugs.openjdk.java.net/browse/JDK-8155043

Webrev:
http://cr.openjdk.java.net/~kbarrett/8155043/webrev.00/

Testing:
RBT runtime and gc nightly.