RISC-V: Refactor and cleanup fma patterns

At present, FMA autovec's patterns do not fully use the corresponding pattern
in vector.md. The previous reason is that the merge operand of pattern in
vector.md cannot be VUNDEF. Now allowing it to be VUNDEF, reunify insn used for
reload pass into vector.md, and the corresponding vlmax pattern in autovec.md
is used for combine. This patch also refactors the corresponding combine
pattern inside autovec-opt.md and removes the unused ones.

gcc/ChangeLog:

	* config/riscv/autovec-opt.md (*<optab>_fma<mode>):
	Removed old combine patterns.
	(*single_<optab>mult_plus<mode>): Ditto.
	(*double_<optab>mult_plus<mode>): Ditto.
	(*sign_zero_extend_fma): Ditto.
	(*zero_sign_extend_fma): Ditto.
	(*double_widen_fma<mode>): Ditto.
	(*single_widen_fma<mode>): Ditto.
	(*double_widen_fnma<mode>): Ditto.
	(*single_widen_fnma<mode>): Ditto.
	(*double_widen_fms<mode>): Ditto.
	(*single_widen_fms<mode>): Ditto.
	(*double_widen_fnms<mode>): Ditto.
	(*single_widen_fnms<mode>): Ditto.
	(*reduc_plus_scal_<mode>): Adjust name.
	(*widen_reduc_plus_scal_<mode>): Adjust name.
	(*dual_widen_fma<mode>): New combine pattern.
	(*dual_widen_fmasu<mode>): Ditto.
	(*dual_widen_fmaus<mode>): Ditto.
	(*dual_fma<mode>): Ditto.
	(*single_fma<mode>): Ditto.
	(*dual_fnma<mode>): Ditto.
	(*single_fnma<mode>): Ditto.
	(*dual_fms<mode>): Ditto.
	(*single_fms<mode>): Ditto.
	(*dual_fnms<mode>): Ditto.
	(*single_fnms<mode>): Ditto.
	* config/riscv/autovec.md (fma<mode>4):
	Reafctor fma pattern.
	(*fma<VI:mode><P:mode>): Removed.
	(fnma<mode>4): Reafctor.
	(*fnma<VI:mode><P:mode>): Removed.
	(*fma<VF:mode><P:mode>):  Removed.
	(*fnma<VF:mode><P:mode>):  Removed.
	(fms<mode>4):  Reafctor.
	(*fms<VF:mode><P:mode>):  Removed.
	(fnms<mode>4): Reafctor.
	(*fnms<VF:mode><P:mode>): Removed.
	* config/riscv/riscv-protos.h (prepare_ternary_operands):
	Adjust prototype.
	* config/riscv/riscv-v.cc (prepare_ternary_operands): Refactor.
	* config/riscv/vector.md (*pred_mul_plus<mode>_undef): New pattern.
	(*pred_mul_plus<mode>): Removed.
	(*pred_mul_plus<mode>_scalar): Removed.
	(*pred_mul_plus<mode>_extended_scalar): Removed.
	(*pred_minus_mul<mode>_undef):  New pattern.
	(*pred_minus_mul<mode>): Removed.
	(*pred_minus_mul<mode>_scalar): Removed.
	(*pred_minus_mul<mode>_extended_scalar): Removed.
	(*pred_mul_<optab><mode>_undef):  New pattern.
	(*pred_mul_<optab><mode>): Removed.
	(*pred_mul_<optab><mode>_scalar): Removed.
	(*pred_mul_neg_<optab><mode>_undef):  New pattern.
	(*pred_mul_neg_<optab><mode>): Removed.
	(*pred_mul_neg_<optab><mode>_scalar): Removed.