Add a minimal, deterministic regression test that reproduces the scenario addressed by JDK-8303513. Currently the only way to exercise the relevant C2 path is running multiple instances of the long-running RunThese8M test using a specific configuration.

To reproduce the failure fixed in JDK-8303513, the test case needs to trigger a C2 compilation of a fully-expanded reflective array creation (java.lang.reflect.Array.newInstance) followed by a fully-expanded checkcast. The entire array creation and checkcast pattern needs to be found to be unreachable during IGVN, and optimized away in a specific order (first the fast path of the java.lang.reflect.Array.newInstance implementation, and then the slow path).