i386: Cleanup V2QI arithmetic instructions

2022-01-13  Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog:

	* config/i386/mmx.md (negv2qi): Disparage GPR alternative a bit.
	Disable for TARGET_PARTIAL_REG_STALL unless optimizing for size.
	(negv2qi splitters): Use lowpart_subreg instead of
	gen_lowpart to create subreg.
	(<plusminus:insn>v2qi3): Disparage GPR alternative a bit.
	Disable for TARGET_PARTIAL_REG_STALL unless optimizing for size.
	(<plusminus:insn>v2qi3 splitters): Use lowpart_subreg instead of
	gen_lowpart to create subreg.
	* config/i386/i386.md (*subqi_ext<mode>_2): Move.