Fix ICE caused by NULL_RTX returned by lowpart_subreg.

Fix ICE caused by NULL_RTX returned by lowpart_subreg.

In validate_subreg, both (subreg:V2HF (reg:SI) 0)
and (subreg:V8HF (reg:V2HF) 0) are valid, but not
for (subreg:V8HF (reg:SI) 0) which causes ICE.

Ideally it should be handled in validate_subreg to support
subreg for all modes available in TARGET_CAN_CHANGE_MODE_CLASS, but
that would be too risky in stage4, so the patch is a walkround in the
backend to force_reg operands before lowpart_subreg for expanders or
pre_reload splitters.

gcc/ChangeLog:

	PR target/104976
	* config/i386/sse.md (ssePSmodelower): New.
	(*avx_cmp<mode>3_ltint_not): Force_reg operand before
	lowpart_subreg to avoid NULL_RTX.
	(<avx512>_fmaddc_<mode>_mask1<round_expand_name>,
	<avx512>_fcmaddc_<mode>_mask1<round_expand_name>,
	fma_<mode>_fmaddc_bcst, fma_<mode>_fcmaddc_bcst,
	<avx512>_<complexopname>_<mode>_mask<round_name>,
	avx512fp16_fcmaddcsh_v8hf_mask1<round_expand_name>,
	avx512fp16_fcmaddcsh_v8hf_mask3<round_expand_name>,
	avx512fp16_fmaddcsh_v8hf_mask3<round_expand_name>,
	avx512fp16_fmaddcsh_v8hf_mask3<round_expand_name>,
	float<floatunssuffix><mode>v4hf2,
	float<floatunssuffix>v2div2hf2,
	fix<fixunssuffix>_truncv4hf<mode>2,
	fix<fixunssuffix>_truncv2hfv2di2, extendv4hf<mode>2,
	extendv2hfv2df2,
	trunc<mode>v4hf2,truncv2dfv2hf2,
	*avx512bw_permvar_truncv16siv16hi_1,
	*avx512bw_permvar_truncv16siv16hi_1_hf,
	*avx512f_permvar_truncv8siv8hi_1,
	*avx512f_permvar_truncv8siv8hi_1_hf,
	*avx512f_vpermvar_truncv8div8si_1,
	*avx512f_permvar_truncv32hiv32qi_1,
	*avx512f_permvar_truncv16hiv16qi_1,
	*avx512f_permvar_truncv4div4si_1,
	*avx512f_pshufb_truncv8hiv8qi_1,
	*avx512f_pshufb_truncv4siv4hi_1,
	*avx512f_pshufd_truncv2div2si_1,
	sdot_prod<mode>, avx2_pblend<ssemodesuffix>_1,
	ashrv2di3,ashrv2di3,usdot_prod<mode>): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr104976.c: New test.
	* gcc.target/i386/avx512fp16-vfcmaddcph-1a.c: Scan either
	vblendps or masked vmovaps.
	* gcc.target/i386/avx512fp16-vfmaddcph-1a.c: Ditto
	* gcc.target/i386/avx512fp16vl-vfcmaddcph-1a.c: Ditto.
	* gcc.target/i386/avx512fp16vl-vfmaddcph-1a.c: Ditto.