Add InstructionFormat.imm_fields.

Consolidate the imm_members and imm_kinds into this list so the
FormatField is the single definition of these properties.

This makes it easier to access the precomputed FormatFields
parametrically, avoiding going through getattr().

This is better for type checking too.

lib/cretonne/meta/gen_instr.py

@@ -515,8 +515,8 @@ def gen_format_constructor(iform, fmt):
         result_type = 'result_type'
 
     # Normal operand arguments. Start with the immediate operands.
-    for kind, name in zip(iform.imm_kinds, iform.imm_members):
-        args.append('{}: {}'.format(name, kind.rust_type))
+    for f in iform.imm_fields:
+        args.append('{}: {}'.format(f.member, f.kind.rust_type))
     # Then the value operands.
     if iform.has_value_list:
         # Take all value arguments as a finished value list. The value lists
@@ -557,8 +557,8 @@ def gen_member_inits(iform, fmt):
 
     # Immediate operands.
     # We have local variables with the same names as the members.
-    for member in iform.imm_members:
-        fmt.line('{}: {},'.format(member, member))
+    for f in iform.imm_fields:
+        fmt.line('{}: {},'.format(f.member, f.member))
 
     # Value operands.
     if iform.has_value_list: