x86: "ssemuladd" adjustments

They're all VEX3- (also covering XOP) or EVEX-encoded. Express that in
the default calculation of "prefix". FMA4 insns also all have a 1-byte
immediate operand.

Where the default calculation is not sufficient / applicable, add
explicit "prefix" attributes. While there also add a "mode" attribute to
fma_<complexpairopname>_<mode>_pair.

gcc/

	* config/i386/i386.md (isa): Move up.
	(length_immediate): Handle "fma4".
	(prefix): Handle "ssemuladd".
	* config/i386/sse.md (*fma_fmadd_<mode>): Add "prefix" attribute.
	(<sd_mask_codefor>fma_fmadd_<mode><sd_maskz_name><round_name>):
	Likewise.
	(<avx512>_fmadd_<mode>_mask<round_name>): Likewise.
	(<avx512>_fmadd_<mode>_mask3<round_name>): Likewise.
	(<sd_mask_codefor>fma_fmsub_<mode><sd_maskz_name><round_name>):
	Likewise.
	(<avx512>_fmsub_<mode>_mask<round_name>): Likewise.
	(<avx512>_fmsub_<mode>_mask3<round_name>): Likewise.
	(*fma_fnmadd_<mode>): Likewise.
	(<sd_mask_codefor>fma_fnmadd_<mode><sd_maskz_name><round_name>):
	Likewise.
	(<avx512>_fnmadd_<mode>_mask<round_name>): Likewise.
	(<avx512>_fnmadd_<mode>_mask3<round_name>): Likewise.
	(<sd_mask_codefor>fma_fnmsub_<mode><sd_maskz_name><round_name>):
	Likewise.
	(<avx512>_fnmsub_<mode>_mask<round_name>): Likewise.
	(<avx512>_fnmsub_<mode>_mask3<round_name>): Likewise.
	(<sd_mask_codefor>fma_fmaddsub_<mode><sd_maskz_name><round_name>):
	Likewise.
	(<avx512>_fmaddsub_<mode>_mask<round_name>): Likewise.
	(<avx512>_fmaddsub_<mode>_mask3<round_name>): Likewise.
	(<sd_mask_codefor>fma_fmsubadd_<mode><sd_maskz_name><round_name>):
	Likewise.
	(<avx512>_fmsubadd_<mode>_mask<round_name>): Likewise.
	(<avx512>_fmsubadd_<mode>_mask3<round_name>): Likewise.
	(*fmai_fmadd_<mode>): Likewise.
	(*fmai_fmsub_<mode>): Likewise.
	(*fmai_fnmadd_<mode><round_name>): Likewise.
	(*fmai_fnmsub_<mode><round_name>): Likewise.
	(avx512f_vmfmadd_<mode>_mask<round_name>): Likewise.
	(avx512f_vmfmadd_<mode>_mask3<round_name>): Likewise.
	(avx512f_vmfmadd_<mode>_maskz_1<round_name>): Likewise.
	(*avx512f_vmfmsub_<mode>_mask<round_name>): Likewise.
	(avx512f_vmfmsub_<mode>_mask3<round_name>): Likewise.
	(*avx512f_vmfmsub_<mode>_maskz_1<round_name>): Likewise.
	(avx512f_vmfnmadd_<mode>_mask<round_name>): Likewise.
	(avx512f_vmfnmadd_<mode>_mask3<round_name>): Likewise.
	(avx512f_vmfnmadd_<mode>_maskz_1<round_name>): Likewise.
	(*avx512f_vmfnmsub_<mode>_mask<round_name>): Likewise.
	(*avx512f_vmfnmsub_<mode>_mask3<round_name>): Likewise.
	(*avx512f_vmfnmsub_<mode>_maskz_1<round_name>): Likewise.
	(*fma4i_vmfmadd_<mode>): Likewise.
	(*fma4i_vmfmsub_<mode>): Likewise.
	(*fma4i_vmfnmadd_<mode>): Likewise.
	(*fma4i_vmfnmsub_<mode>): Likewise.
	(fma_<complexopname>_<mode><sdc_maskz_name><round_name>): Likewise.
	(<avx512>_<complexopname>_<mode>_mask<round_name>): Likewise.
	(avx512fp16_fma_<complexopname>sh_v8hf<mask_scalarcz_name><round_scalarcz_name>):
	Likewise.
	(avx512fp16_<complexopname>sh_v8hf_mask<round_name>): Likewise.
	(xop_p<macs><ssemodesuffix><ssemodesuffix>): Likewise.
	(xop_p<macs>dql): Likewise.
	(xop_p<macs>dqh): Likewise.
	(xop_p<macs>wd): Likewise.
	(xop_p<madcs>wd): Likewise.
	(fma_<complexpairopname>_<mode>_pair): Likewise. Add "mode" attribute.