RFR: 8294014: Remove redundant UseCompiler conditions

[Julian Waters]

Several areas that use CompileBroker::should_compile_new_jobs() also contain a UseCompiler check which isn't needed, since the aforementioned already contains a check for this flag. In some areas (Such as with compileBroker.cpp which has AlwaysCompileLoopMethods) this duplicated check is required, so we can mark should_compile_new_jobs() and the related get_compilation_activity_mode() as inline to make the obvious optimization more obvious to the compiler used in the build (This should be fine, since both are small methods).

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Commit messages:
 - CompileBroker methods to inline
 - Remove redundant condition in compilationPolicy

Changes: https://git.openjdk.org/jdk/pull/10342/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10342&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294014
  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10342.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10342/head:pull/10342

PR: https://git.openjdk.org/jdk/pull/10342