Split EntityMap into entity::PrimaryMap and entity::EntityMap.

The new PrimaryMap replaces the primary EntityMap and the PrimaryEntityData
marker trait which was causing some confusion. We now have a clear
division between the two types of maps:

- PrimaryMap is used to assign entity numbers to the primary data for an
  entity.
- EntityMap is a secondary mapping adding additional info.

The split also means that the secondary EntityMap can now behave as if
all keys have a default value. This means that we can get rid of the
annoying ensure() and get_or_default() methods ther were used everywhere
instead of indexing. Just use normal indexing now; non-existent keys
will return the default value.

lib/cretonne/src/dominator_tree.rs

@@ -1,6 +1,6 @@
 //! A Dominator Tree represented as mappings of Ebbs to their immediate dominator.
 
-use entity_map::EntityMap;
+use entity::EntityMap;
 use flowgraph::{ControlFlowGraph, BasicBlock};
 use ir::{Ebb, Inst, Function, Layout, ProgramOrder, ExpandedProgramPoint};
 use packed_option::PackedOption;
@@ -339,7 +339,7 @@ impl DominatorTree {
     pub fn recompute_split_ebb(&mut self, old_ebb: Ebb, new_ebb: Ebb, split_jump_inst: Inst) {
         if !self.is_reachable(old_ebb) {
             // old_ebb is unreachable, it stays so and new_ebb is unreachable too
-            *self.nodes.ensure(new_ebb) = Default::default();
+            self.nodes[new_ebb] = Default::default();
             return;
         }
         // We use the RPO comparison on the postorder list so we invert the operands of the
@@ -350,7 +350,7 @@ impl DominatorTree {
                 .binary_search_by(|probe| self.rpo_cmp_ebb(old_ebb, *probe))
                 .expect("the old ebb is not declared to the dominator tree");
         let new_ebb_rpo = self.insert_after_rpo(old_ebb, old_ebb_postorder_index, new_ebb);
-        *self.nodes.ensure(new_ebb) = DomNode {
+        self.nodes[new_ebb] = DomNode {
             rpo_number: new_ebb_rpo,
             idom: Some(split_jump_inst).into(),
         };