Convert return formats to value lists.

With the Return and ReturnReg formats converted to using value lists for
storing their arguments, thee are no remaining instruction formats with
variable argument lists in boxed storage.

The Return and ReturnReg formats are also going to be merged since
they are identical now.

lib/cretonne/meta/base/formats.py

@@ -51,8 +51,8 @@ Call = InstructionFormat(
 IndirectCall = InstructionFormat(
         sig_ref, VALUE, VARIABLE_ARGS,
         multiple_results=True, value_list=True)
-Return = InstructionFormat(VARIABLE_ARGS, boxed_storage=True)
-ReturnReg = InstructionFormat(VALUE, VARIABLE_ARGS, boxed_storage=True)
+Return = InstructionFormat(VARIABLE_ARGS, value_list=True)
+ReturnReg = InstructionFormat(VALUE, VARIABLE_ARGS, value_list=True)
 
 # Finally extract the names of global variables in this module.
 InstructionFormat.extract_names(globals())

lib/cretonne/meta/cdsl/formats.py

@@ -63,6 +63,10 @@ class InstructionFormat(object):
         self.value_operands = tuple(
                 i for i, k in enumerate(self.kinds) if k is VALUE)
 
+        # We require a value list for storage of variable arguments.
+        if VARIABLE_ARGS in self.kinds:
+            assert self.has_value_list, "Need a value list for variable args"
+
         # The typevar_operand argument must point to a 'value' operand.
         self.typevar_operand = kwargs.get('typevar_operand', None)  # type: int
         if self.typevar_operand is not None: