Add custom legalization for floating point constants.

Use the simplest expansion which materializes the bits of the floating
point constant as an integer and then bit-casts to the floating point
type. In the future, we may want to use constant pools instead. Either
way, we need custom legalization.

Also add a legalize_monomorphic() function to the Python targetISA class
which permits the configuration of a default legalization action for
monomorphic instructions, just like legalize_type() does for polymorphic
instructions.

lib/cretonne/meta/cdsl/isa.py

@@ -263,6 +263,16 @@ class CPUMode(object):
             ty = ValueType.by_name(name)
             self.type_legalize[ty] = xgrp
 
+    def legalize_monomorphic(self, xgrp):
+        # type: (XFormGroup) -> None
+        """
+        Configure the legalization action to take for monomorphic instructions
+        which don't have a controlling type variable.
+
+        See also `legalize_type()` for polymorphic instructions.
+        """
+        self.type_legalize[None] = xgrp
+
     def get_legalize_action(self, ty):
         # type: (ValueType) -> XFormGroup
         """