The prolog in BytecodeInterpreter is hairy due to early initialization of interpreter state. Previous rewrites make it mostly redundant, and we can now simplify it.

This also implicitly fixes a initialization bug. If JvmtiExport::can_post_interpreter_events() changes at runtime, we will call into the uninitialized version:

    // Call the interpreter
    if (JvmtiExport::can_post_interpreter_events()) {
      BytecodeInterpreter::run<true>(istate);
    } else {
      BytecodeInterpreter::run<false>(istate);
    }