Merge vec_addsub patterns

This merges the vec_addsub<mode>3 patterns using a mode attribute
for the vec_merge merge operand.

2021-06-18  Richard Biener  <rguenther@suse.de>

	* config/i386/sse.md (vec_addsubv4df3, vec_addsubv2df3,
	vec_addsubv8sf3, vec_addsubv4sf3): Merge into ...
	(vec_addsub<mode>3): ... using a new addsub_cst mode attribute.