i386: Fix and improve insn constraint for V2QI arithmetic/shift insns

i386: Fix and improve insn constraint for V2QI arithmetic/shift insns

optimize_function_for_size_p predicate is not stable during optab selection,
because it also depends on node->count/node->frequency of the current function,
which are updated during IPA, so they may change between early opts and
late opts.  Use optimize_size instead - optimize_size implies
optimize_function_for_size_p (cfun), so if a named pattern uses
"&& optimize_size" and the insn it splits into uses
optimize_function_for_size_p (cfun), it shouldn't fail.

	PR target/114232

gcc/ChangeLog:

	* config/i386/mmx.md (negv2qi2): Enable for optimize_size instead
	of optimize_function_for_size_p.  Explictily enable for TARGET_SSE2.
	(negv2qi SSE reg splitter): Enable for TARGET_SSE2 only.
	(<plusminus:insn>v2qi3): Enable for optimize_size instead
	of optimize_function_for_size_p.  Explictily enable for TARGET_SSE2.
	(<plusminus:insn>v2qi SSE reg splitter): Enable for TARGET_SSE2 only.
	(<any_shift:insn>v2qi3): Enable for optimize_size instead
	of optimize_function_for_size_p.