Refactor validator so you can write good unit tests for dominance calculations.

E.g. construct binary module, then compute dominators, then access immediate_dominators for particular blocks.

Start by locating the validator's dominance calculation entry point and any existing tests; the issue names no files or test targets. Review how a binary module is constructed, dominators are computed, and immediate_dominators is accessed for blocks. Done means those steps can be exercised cleanly in focused unit tests.

Written by the indexing model from the issue text.