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/isa/intel/encodings.py

@@ -20,6 +20,7 @@ except ImportError:
     pass
 
 
+I32.legalize_monomorphic(expand)
 I32.legalize_type(
         default=narrow,
         b1=expand,
@@ -27,6 +28,7 @@ I32.legalize_type(
         f32=expand,
         f64=expand)
 
+I64.legalize_monomorphic(expand)
 I64.legalize_type(
         default=narrow,
         b1=expand,