Add an EntityMap::get_or_default() method.

This covers a common pattern for secondary entity maps: Get the value in
the map or the default value for out-of-range keys.

lib/cretonne/src/write.rs

@@ -189,13 +189,7 @@ fn write_instruction(w: &mut Write,
             if !func.locations.is_empty() {
                 let regs = isa.register_info();
                 for &r in func.dfg.inst_results(inst) {
-                    write!(s,
-                           ",{}",
-                           func.locations
-                               .get(r)
-                               .cloned()
-                               .unwrap_or_default()
-                               .display(&regs))?
+                    write!(s, ",{}", func.locations.get_or_default(r).display(&regs))?
                 }
             }
             write!(s, "]")?;