Move ensure_domtree out of Context and into DominatorTree.

This also moves the calls to it out of Context and into the passes that
actually need it, so that Context's functions don't have any logic of
their own.

lib/cretonne/src/licm.rs

@@ -16,6 +16,7 @@ pub fn do_licm(
     domtree: &mut DominatorTree,
     loop_analysis: &mut LoopAnalysis,
 ) {
+    domtree.ensure(func, cfg);
     loop_analysis.compute(func, cfg, domtree);
     for lp in loop_analysis.loops() {
         // For each loop that we want to optimize we determine the set of loop-invariant