RFR: 8219922: Simplify IndexSetIterator::next using count_trailing_zeros

[Claes Redestad]

Hi,

using count_trailing_zeros to implement IndexSetIterator::next
simplifies the code and shows a small speed-up in profiles.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8219922
Webrev: http://cr.openjdk.java.net/~redestad/8219922/open.00/

Testing: tier1+2

Thanks!

/Claes