RFR: 8178352: BitMap::get_next_zero_offset may give wrong result on Mac

[Kim Barrett]

Please review this change to BitMap::get_next_zero_offset_inline to
remove its use of left_n_bits, thereby avoiding
https://bugs.openjdk.java.net/browse/JDK-8178348.

CR:
https://bugs.openjdk.java.net/browse/JDK-8178352

Webrev:
http://cr.openjdk.java.net/~kbarrett/8178352/hotspot.00/

Testing:
JPRT, with the test from
https://bugs.openjdk.java.net/browse/JDK-8169039
included in the tested repository.