mmix.h: Improve comments.

mmix.h: Improve comments.

	* config/mmix/mmix.h: Improve comments.
	(CPP_SPEC, CPP_PREDEFINES): Don't define.
	(TARGET_CPU_CPP_BUILTINS): Define.
	(CANONICALIZE_COMPARISON): Don't define, replace with comment.
	(PREDICATE_CODES) <"mmix_reg_or_8bit_or_256_operand">: Remove
	unused predicate.
	* config/mmix/mmix.c (MMIX_OUTPUT_REGNO): Add cast to avoid
	compiler warning.
	(mmix_constant_address_p): Remove another
	redundant test before case.
	(mmix_canonicalize_comparison): Remove unused function.
	(mmix_print_operand_address): Don't test and adjust for operand in
	non-canonical format.
	(mmix_reg_or_8bit_or_256_operand): Remove unused predicate.
	(mmix_gen_compare_reg): Don't use CANONICALIZE_COMPARISON.

From-SVN: r54474