RFR: 8256883: C2: Add a RegMask iterator [v4]

[Per Liden]

On Tue, 24 Nov 2020 00:14:11 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> By implementing a simple RegMaskIterator we can speed this up and possibly make the code a bit clearer by doing so.
>> 
>> As a data point, this reduce the `C2Compiler::initialize` overhead from 8.82M instructions to 8.58M instructions, from the improvement in `PhaseChaitin::post_allocate_copy_removal` (~16k insns/compilation). The gain varies with type of compilation, so on the naive tests in `SimpleRepeatCompilation` it's in the noise (~2k insns/compilation on `trivialMath, for example).
>
> Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove extra commentary in ZGC aarch code (requested by Per Liden)

ZGC changes look good.

-------------

Marked as reviewed by pliden (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1397