[ImplicitNullChecks] NFC: Refactor dependence safety check

After computing dependence, we check if it is safe to hoist by
identifying if it clobbers any liveIns in the sibling block (NullSucc).
This check is moved to its own function which will be used in the
soon-to-be modified dependence checking algorithm for implicit null
checks pass.

Tests-Run: lit tests on X86/implicit-*