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/context.rs

@@ -115,13 +115,6 @@ impl Context {
         self.domtree.compute(&self.func, &self.cfg);
     }
 
-    /// Ensure that a valid domtree exists.
-    pub fn ensure_domtree(&mut self) {
-        if !self.domtree.is_valid() {
-            self.domtree.compute(&self.func, &self.cfg);
-        }
-    }
-
     /// Perform simple GVN on the function.
     pub fn simple_gvn(&mut self) -> CtonResult {
         do_simple_gvn(&mut self.func, &mut self.cfg);
@@ -132,7 +125,6 @@ impl Context {
 
     /// Perform LICM on the function.
     pub fn licm(&mut self) -> CtonResult {
-        self.ensure_domtree();
         do_licm(
             &mut self.func,
             &mut self.cfg,
@@ -144,12 +136,11 @@ impl Context {
 
     /// Run the register allocator.
     pub fn regalloc(&mut self, isa: &TargetIsa) -> CtonResult {
-        self.ensure_domtree();
         self.regalloc.run(
             isa,
             &mut self.func,
             &self.cfg,
-            &self.domtree,
+            &mut self.domtree,
         )
     }