Add a BinaryOverflow instruction format.

This will eventualy be used for add-with-carry and add-with-overflow type
instructions. For now it only serves as a representative of instruction formats
that have multiple_results=True and boxed_storage=False at the same time.

meta/cretonne/formats.py

@@ -23,6 +23,9 @@ Binary = InstructionFormat(value, value)
 BinaryImm = InstructionFormat(value, imm64)
 BinaryImmRev = InstructionFormat(imm64, value)
 
+# Generate result + overflow flag.
+BinaryOverflow = InstructionFormat(value, value, multiple_results=True)
+
 Jump = InstructionFormat(ebb, variable_args, boxed_storage=True)
 Branch = InstructionFormat(value, ebb, variable_args, boxed_storage=True)
 BranchTable = InstructionFormat(value, jump_table)