ARM64: Optimize RegList::IsValid() to speed up simulation in (opt)debug mode.

On my machine this improves the simulation speed noticeably.
| test               | base  | patched |
|--------------------|-------|---------|
| optdebug           | 15:37 | 11:07   |
| debug --quickcheck | 33:53 | 26:14   |

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/209353010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20232 ce2b1a6d-e550-0410-aec6-3dcde31c8c00