Remove redundant UseCompiler conditions

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).