Python InstructionFormat refactoring.

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.

lib/cretonne/meta/gen_legalizer.py

@@ -46,9 +46,8 @@ def unwrap_inst(iref, node, fmt):
             fmt.line('ref data,')
         else:
             # Fields are encoded directly.
-            for m in iform.members:
-                if m:
-                    fmt.line('{},'.format(m))
+            for m in iform.imm_members:
+                fmt.line('{},'.format(m))
             if nvops == 1:
                 fmt.line('arg,')
             elif nvops > 1: