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/base/legalize.py

@@ -27,6 +27,13 @@ narrow = XFormGroup('narrow', """
         operations are expressed in terms of smaller integer types.
         """)
 
+widen = XFormGroup('widen', """
+        Legalize instructions by widening.
+
+        The transformations in the 'widen' group work by expressing
+        instructions in terms of larger types.
+        """)
+
 expand = XFormGroup('expand', """
         Legalize instructions by expansion.