RFR: JDK-8267916: Make as_CompilerThread consistent with other as_*_thread methods [v2]

[Ioi Lam]

On Wed, 2 Jun 2021 14:17:58 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Simple refactoring around `as_CompilerThread`. A new file, `compilerThread.inline.hpp`, is created to get around the circular dependency.
>
> Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   cast

This looks good to me. If we decide to use Compiler::cast, the RFE title should be updated before integration.

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Marked as reviewed by iklam (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/4240