RFR: 8251322: Improve BitMap::iterate

[Kim Barrett]

Please review this improvement to BitMap::iterate.  The code is both
simplified and made more performant by using get_next_one_bit to scan
the bitmap.  Making it inline may allow the compiler to devirtualize
and possibly inline the closure invocation.  These changes result in
some configurations executing ~1M fewer instructions during startup.

CR:
https://bugs.openjdk.java.net/browse/JDK-8251322

Webrev:
https://cr.openjdk.java.net/~kbarrett/8251322/open.00/

Testing:
mach5 tier1-3.
Used Linux-x64 perf on java -version to examine startup impact.