Move verify calls out of Context.

Also, move flowgraph() calls out of filetest and into the passes that
need them so that filetest doesn't have embedded knowledge of these
dependencies.

This resolves a TODO about the way Context was running the verifier, and
it makes the Context functions and the filetest runners more transparent.

This also fixes simple_gvn to use the existing dominator tree rather
than computing its own.

lib/cretonne/src/dominator_tree.rs

@@ -231,8 +231,7 @@ impl DominatorTree {
     ///
     /// Note that this doesn't perform any kind of validity checks. It simply checks if the
     /// `compute()` method has been called since the last `clear()`. It does not check that the
-    /// dominator tree is consistent
-    /// with the CFG>
+    /// dominator tree is consistent with the CFG.
     pub fn is_valid(&self) -> bool {
         !self.nodes.is_empty()
     }