RFR: 8217629: RegMask::find_lowest_bit can reuse count_trailing_zeros utility

[Claes Redestad]

Hi,

reusing the count_trailing_zeros utility from RegMask is a simple
cleanup which may enable optimizations on many platforms, like tzcnt
on Intel/AMD, and improves inlining.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8217629
Webrev: http://cr.openjdk.java.net/~redestad/8217629/open.00/

On my startup tests and profiles this reduces instructions spent in C2s
register allocator by ~4%, and ~2% on the total.

Testing: tier1-3

Thanks!

/Claes