AVR: Rework patterns that add / subtract an (inverted) MSB.

AVR: Rework patterns that add / subtract an (inverted) MSB.

gcc/
	* config/avr/avr-protos.h (avr_out_add_msb): New proto.
	* config/avr/avr.cc (avr_out_add_msb): New function.
	(avr_adjust_insn_length) [ADJUST_LEN_ADD_GE0,
	ADJUST_LEN_ADD_LT0]: Handle cases.
	* config/avr/avr.md (adjust_len) <add_lt0, add_ge0>: New attr values.
	(QISI2): New mode iterator.
	(C_MSB): New mode_attr.
	(*add<mode>3...msb_split, *add<mode>3.ge0, *add<mode>3.lt0)
	(*sub<mode>3...msb_split, *sub<mode>3.ge0, *sub<mode>3.lt0): New
	patterns replacing old ones, but with iterators and
	using avr_out_add_msb() for asm out.