Make some changes that will make it easier to get rid of the
'value_operands' and 'members' fields in the Python InstructionFormat
class. This is necessary to be able to combine instruction formats that
all use a value list representation, but with different fixed value
operands. The goal is to eventually identify formats by a new signature:
(multiple_results, imm_kinds, num_value_operands)
Start by adding new fields:
- imm_members and imm_kinds are lists describing the format operands,
excluding any values and variable_args operands.
- num_value_operands is the number of fixed value operands, or None in a
has_value-list format.
Use these new members in preference to the old ones where possible.
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