i386: Add vec_fm{addsub,subadd}v2sf4 patterns [PR116979]

As mentioned in the PR, the addition of vec_addsubv2sf3 expander caused
the testcase to be vectorized and no longer to use fma.
The following patch adds new expanders so that it can be vectorized
again with the alternating add/sub fma instructions.

There is some bug on the slp cost computation side which causes it
not to count some scalar multiplication costs, but I think the patch
is desirable anyway before that is fixed and the testcase for now just
uses -fvect-cost-model=unlimited.

2024-12-13  Jakub Jelinek  <jakub@redhat.com>

	PR target/116979
	* config/i386/mmx.md (vec_fmaddsubv2sf4, vec_fmsubaddv2sf4): New
	define_expand patterns.

	* gcc.target/i386/pr116979.c: New test.