Make legalization actions configurable.

When an instruction doesn't have a valid encoding for the target ISA, it
needs to be legalized. Different legalization strategies can be
expressed as separate XFormGroup objects.

Make the choice of XFormGroup configurable per CPU mode, rather than
depending on a hard-coded default.

Add a CPUMode.legalize_type() method which assigns an XFormGroup to
controlling type variables and lets you set a default.

Add a `legalize` field to Level1Entry so the first-level hash table
lookup gives us the configured default legalization action for the
instruction's controlling type variable.

lib/cretonne/meta/cdsl/xform.py

@@ -241,6 +241,10 @@ class XFormGroup(object):
         self.name = name
         self.__doc__ = doc
 
+    def __str__(self):
+        # type: () -> str
+        return self.name
+
     def legalize(self, src, dst):
         # type: (Union[Def, Apply], Rtl) -> None
         """