RFR: 8220159: Optimize various RegMask operations by introducing watermarks

[Claes Redestad]

Hi,

by introducing a low and high water mark of the RegMask words that we
are sure have register bits, we can reduce time spent doing a variety of
operations.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8220159
Webrev: http://cr.openjdk.java.net/~redestad/8220159/open.00/

On a few profiled startup application the average number of instructions
spent compiling methods in C2 drops 10-15%, and we see improvements on a
range of startup and footprint applications.

There are also a few improvements on microbenchmarks, likely due to a
positive effect on warmup times.

Testing: tier1-3

Thanks!

/Claes