Disable gathers for znver3 for vectors with 2 or 4 elements

gcc/ChangeLog:

2022-03-28  Jan Hubicka  <hubicka@ucw.cz>

	* config/i386/i386-builtins.cc (ix86_vectorize_builtin_gather): Test
	TARGET_USE_GATHER_2PARTS and TARGET_USE_GATHER_4PARTS.
	* config/i386/i386.h (TARGET_USE_GATHER_2PARTS): New macro.
	(TARGET_USE_GATHER_4PARTS): New macro.
	* config/i386/x86-tune.def (X86_TUNE_USE_GATHER_2PARTS): New tune
	(X86_TUNE_USE_GATHER_4PARTS): New tune