i386: Fix up signbit<mode>2 expander [PR112816]

The following testcase ICEs, because the signbit<mode>2 expander uses an
explicit SUBREG in the pattern around match_operand with register_operand
predicate.  If we are unlucky enough that expansion tries to expand it
with some SUBREG as operands[1], we have two nested SUBREGs in the IL,
which is not valid and causes ICE later.

2023-12-04  Jakub Jelinek  <jakub@redhat.com>

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

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