Fix two precedence-related regressions.

Fix two precedence-related regressions that resulted from converting
equivalence and dominator sets from lists to true sets:

1) When merging two equivalent precedences, replace equiv/dominators for
   all precedences in both sets, rather than assuming that the set being
   merged has only one element.
2) Fix a run-time error when generating a graphviz representation of the
   precedence graph.