Add a regmove instruction.

This will be used to locally change the register locations of values in
order to satisfy instruction constraints.

lib/cretonne/src/ir/dfg.rs

@@ -687,7 +687,7 @@ impl<'a> fmt::Display for DisplayInst<'a> {
         } else {
             write!(f, "{}.{}", inst.opcode(), typevar)?;
         }
-        write_operands(f, dfg, self.1)
+        write_operands(f, dfg, None, self.1)
     }
 }