CodeCache::mark_for_deoptimization should avoid verifying dependencies multiple times

The dependency verification code in CodeCache::mark_for_deoptimization walks over all live nmethods in the code cache and checks all dependencies for each nmethod. This leads to checking the same dependency multiple times which can take a huge amount of time.

Due to this fact some tests time out when running with debug builds (see JDK-7194662). Especially tests with a lot of nmethods in the code cache.