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/wasm.rs

@@ -158,6 +158,8 @@ fn handle_module(
             context.verify(None).map_err(|err| {
                 pretty_verifier_error(&context.func, None, err)
             })?;
+            context.flowgraph();
+            context.compute_loop_analysis();
             context.licm();
             context.verify(None).map_err(|err| {
                 pretty_verifier_error(&context.func, None, err)