ISLE: explicitly define rule-ordering heuristic

Right now, the ISLE compiler prioritizes matching "more specific rules first" when building the decision trie that becomes the generated code.

This is somewhat intuitive, but also imprecise. The actual details depend on subtle workings of the trie construction. We should:

1. Define a proper total ordering over all patterns, which can be understood independently of compiler implementation details and reasoned about intuitively; and
2. Implement this in place of the current system.