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.

cranelift/src/filetest/simple_gvn.rs

@@ -38,6 +38,7 @@ impl SubTest for TestSimpleGVN {
         let mut comp_ctx = cretonne::Context::new();
         comp_ctx.func = func.into_owned();
 
+        comp_ctx.flowgraph();
         comp_ctx.simple_gvn();
         comp_ctx.verify(context.isa).map_err(|e| {
             pretty_error(&comp_ctx.func, context.isa, Into::into(e))