Create FormatField attributes on demand.
Create FormatField attributes on demand.

The InstructionFormat objects make their non-value operands available as
FormatField attributes for use by predicates etc.

Compute these on demand instead of up front. This makes it possible for
the mypy tool to infer the types of these attributes from the
__getattr__ signature.

sunfishcode/cretonne,stoklund/cretonne,stoklund/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne