Add tests that exercise C2's generational ZGC barrier elision, including different combinations of the following dimensions:

- elision type (allocation vs. domination)
- first access type (read vs. write)
- second access type (read vs. write)
- scope (local vs. global)
- object type (array vs. class instance)
- compile-time offset knownness (for arrays only)
- object equality
- field equality
- ...