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/simple_gvn.rs

@@ -14,8 +14,8 @@ fn trivially_unsafe_for_gvn(opcode: Opcode) -> bool {
 /// Perform simple GVN on `func`.
 ///
 pub fn do_simple_gvn(func: &mut Function, cfg: &mut ControlFlowGraph, domtree: &mut DominatorTree) {
-    cfg.ensure(func);
-    domtree.ensure(func, cfg);
+    debug_assert!(cfg.is_valid());
+    debug_assert!(domtree.is_valid());
 
     let mut visible_values: HashMap<(InstructionData, Type), Inst> = HashMap::new();