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

[Albert Mingkun Yang]

Simple refactoring around `as_CompilerThread`. A new file, `compilerThread.inline.hpp`, is created to get around the circular dependency.

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Commit messages:
 - compiler_thread

Changes: https://git.openjdk.java.net/jdk/pull/4240/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4240&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267916
  Stats: 64 lines in 13 files changed: 50 ins; 9 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4240.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4240/head:pull/4240

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