Make passes assert their dependencies consistently. (#156)

* Make passes assert their dependencies consistently.

This avoids ambiguity about whose responsibility it is to run
to compute cfg, domtree, and loop_analysis data.

* Reset the `valid` flag in DominatorTree's `clear()`.

* Remove the redundant assert from DominatorTree::with_function.

* Remove the message strings from obvious asserts.

This avoids having them spill out into multiple lines.

* Refactor calls to `compute` on `Context` objects into helper functions.

lib/cretonne/src/regalloc/context.rs

@@ -60,8 +60,7 @@ impl Context {
         cfg: &ControlFlowGraph,
         domtree: &mut DominatorTree,
     ) -> CtonResult {
-        // Ensure that a valid domtree exists.
-        domtree.ensure(func, cfg);
+        debug_assert!(domtree.is_valid());
 
         // `Liveness` and `Coloring` are self-clearing.
         self.virtregs.clear();