RFR: 8256883: C2: Add a RegMask iterator

[Vladimir Kozlov]

On Mon, 23 Nov 2020 18:01:01 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).

There are 2 places in ZGC code (x86 and aarch64) which could use new iterator too:
https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/gc/z/zBarrierSetAssembler_x86.cpp#L482

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Changes requested by kvn (Reviewer).

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