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/legalizer/mod.rs

@@ -33,6 +33,8 @@ use self::heap::expand_heap_addr;
 /// - Fill out `func.encodings`.
 ///
 pub fn legalize_function(func: &mut ir::Function, cfg: &mut ControlFlowGraph, isa: &TargetIsa) {
+    debug_assert!(cfg.is_valid());
+
     boundary::legalize_signatures(func, isa);
 
     func.encodings.resize(func.dfg.num_insts());