A more optimal implementation for finding base references

[qinsoon]

We currently use SideMetadata::find_prev_non_zero_value to find the base reference for an internal reference. The implementation of SideMetadata::find_prev_non_zero_value is fully general for all kinds of side metadata, and does not use any specific assumption about object references. In our discussion for #1192, Steve mentioned that we may be able to implement a more optimal implementation of finding base references. For example, if we assume the returned address will be an aligned object reference, we don't need the filtering in #1192. There might be other opportunities for optimization. We need further discussion to decide what we can optimize.