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/verifier/liveness.rs

@@ -63,7 +63,7 @@ impl<'a> LivenessVerifier<'a> {
     fn check_insts(&self) -> Result {
         for ebb in self.func.layout.ebbs() {
             for inst in self.func.layout.ebb_insts(ebb) {
-                let encoding = self.func.encodings.get_or_default(inst);
+                let encoding = self.func.encodings[inst];
 
                 // Check the defs.
                 for &val in self.func.dfg.inst_results(inst) {

lib/cretonne/src/verifier/mod.rs

@@ -688,7 +688,7 @@ impl<'a> Verifier<'a> {
         for (&arg, &abi) in args.iter().zip(expected_args) {
             // Value types have already been checked by `typecheck_variable_args_iterator()`.
             if let ArgumentLoc::Stack(offset) = abi.location {
-                let arg_loc = self.func.locations.get_or_default(arg);
+                let arg_loc = self.func.locations[arg];
                 if let ValueLoc::Stack(ss) = arg_loc {
                     // Argument value is assigned to a stack slot as expected.
                     self.verify_stack_slot(inst, ss)?;
@@ -811,7 +811,7 @@ impl<'a> Verifier<'a> {
             None => return Ok(()),
         };
 
-        let encoding = self.func.encodings.get_or_default(inst);
+        let encoding = self.func.encodings[inst];
         if encoding.is_legal() {
             let verify_encoding =
                 isa.encode(&self.func.dfg,