i386: Disable use_gather_2parts and use_gather_4parts for Diamond Rapids

DMR architecture achieves optimal pipeline utilization and
parallelism for gather emulation with 2/4-element vector. Adjust the
tune accordingly.

gcc/ChangeLog:

	* config/i386/x86-tune.def (X86_TUNE_USE_GATHER_2PARTS): Disable
	for m_DIAMONDRAPIDS.
	(X86_TUNE_USE_GATHER_4PARTS): Likewise.

(cherry picked from commit 5c556f9f539a9493fea9401f2e106bfd8be9ed5e)