RFR: 8256883: C2: Add a RegMask iterator [v3]

[Claes Redestad]

> 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:

  Rename iterator variables in ZGC code

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1397/files
  - new: https://git.openjdk.java.net/jdk/pull/1397/files/b5345e9e..b928fcc1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1397&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1397&range=01-02

  Stats: 7 lines in 2 files changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1397.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1397/head:pull/1397

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