Add support for type variable wildcards in bound instructions.

Instructions will multiple type variables can now use `any` to indicate
encodings that don't care about the value of a secondary type variable:

    ishl.i32.any instead of ishl.i32.i32

This is only allowed for secondary type variables (which are converted
to instruction predicates). The controlling type variable must still be
fully specified because it is used to key the encoding tables.

lib/cretonne/meta/cdsl/isa.py

@@ -424,6 +424,9 @@ class Encoding(object):
         # Add secondary type variables to the instruction predicate.
         if len(self.typevars) > 1:
             for tv, vt in zip(self.inst.other_typevars, self.typevars[1:]):
+                # A None tv is an 'any' wild card: `ishl.i32.any`.
+                if vt is None:
+                    continue
                 typred = TypePredicate.typevar_check(self.inst, tv, vt)
                 instp = And.combine(instp, typred)