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/verifier/mod.rs

@@ -134,7 +134,9 @@ pub fn verify_context(
     isa: Option<&TargetIsa>,
 ) -> Result {
     let verifier = Verifier::new(func, isa);
-    verifier.cfg_integrity(cfg)?;
+    if cfg.is_valid() {
+        verifier.cfg_integrity(cfg)?;
+    }
     if domtree.is_valid() {
         verifier.domtree_integrity(domtree)?;
     }