i386: Fix mmx.md signbit expanders [PR112816]

Apparently when looking for "signbit<mode>2" vector expanders, I've only
looked at sse.md and forgot mmx.md, which has two further ones and the
following patch still ICEd.

2023-12-19  Jakub Jelinek  <jakub@redhat.com>

	PR target/112816
	* config/i386/mmx.md (signbitv2sf2, signbit<mode>2): Force operands[1]
	into a REG.

	* gcc.target/i386/sse2-pr112816-2.c: New test.