tcg: Remove args_ct from TCGOpDef

tcg: Remove args_ct from TCGOpDef

Introduce a new function, opcode_args_ct, to look up the argument
set for an opcode.  We lose the ability to assert the correctness
of the map from TCGOpcode to constraint sets at startup, but we can
still validate at runtime upon lookup.

Rename process_op_defs to process_constraint_sets, as it now does
nothing to TCGOpDef.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>