RFR: 8235551: BitMap::count_one_bits should use population_count

[Claes Redestad]

Hi,

with some adjustments, BitMap::count_one_bits can be rewritten to use
the population_count utility method introduced in JDK-8217519

Bug:    https://bugs.openjdk.java.net/browse/JDK-8235551
Webrev: http://cr.openjdk.java.net/~redestad/8235551/open.00/

This generalizes population_count to accept any unsigned integer, to
accommodate that BitMap will use 32- or 64-bit integers depending on
platform.

Testing: tier1-3

Thanks!

/Claes