Make sure blackholes are tagged for all JVMCI paths

In post-integration comment to JDK-8259316, Tom says:
 "In this code, 6018336#diff-fa2433a762244542fec57f9d58dd3092bae74f354acf0ef33603a5f8306fd7daR995, the call to CompilerOracle::tag_blackhole_if_possible should be outside of the if. As written, methods won't be properly tagged for libgraal, only when used with the pure Java graal."

See:
 https://github.com/openjdk/jdk/pull/2024#issuecomment-832834830