Previously we essentially never freed block even after invalidation. Their reference count never reached zero for a couple of reasons: 1. `Branch::block` formed a cycle with the block holding the branch 2. Strong count on a branch that has ever contained a stub never reached 0 because we increment the `.clone()` call for `BranchRef::into_raw()` didn't have a matching decrement. It's not safe to immediately deallocate blocks during invalidation since `branch_stub_hit()` can end up running with a branch pointer from an invalidated branch. To plug the s, we wait until code GC or global invalidation and deallocate the blocks for iseqs that are definitely not running.