LoongArch: Simplify {lsx_,lasx_x}vmaddw description

Like what we've done for {lsx_,lasx_x}v{add,sub,mul}l{ev,od}, use
special predicates and TImode RTL instead of hard-coded const vectors
and UNSPECs.

Also reorder two operands of the outer plus in the template, so combine
will recognize {x,}vadd + {x,}vmulw{ev,od} => {x,}vmaddw{ev,od}.

gcc/ChangeLog:

	* config/loongarch/lasx.md (UNSPEC_LASX_XVMADDWEV): Remove.
	(UNSPEC_LASX_XVMADDWEV2): Remove.
	(UNSPEC_LASX_XVMADDWEV3): Remove.
	(UNSPEC_LASX_XVMADDWOD): Remove.
	(UNSPEC_LASX_XVMADDWOD2): Remove.
	(UNSPEC_LASX_XVMADDWOD3): Remove.
	(lasx_xvmaddwev_h_b<u>): Remove.
	(lasx_xvmaddwev_w_h<u>): Remove.
	(lasx_xvmaddwev_d_w<u>): Remove.
	(lasx_xvmaddwev_q_d): Remove.
	(lasx_xvmaddwod_h_b<u>): Remove.
	(lasx_xvmaddwod_w_h<u>): Remove.
	(lasx_xvmaddwod_d_w<u>): Remove.
	(lasx_xvmaddwod_q_d): Remove.
	(lasx_xvmaddwev_q_du): Remove.
	(lasx_xvmaddwod_q_du): Remove.
	(lasx_xvmaddwev_h_bu_b): Remove.
	(lasx_xvmaddwev_w_hu_h): Remove.
	(lasx_xvmaddwev_d_wu_w): Remove.
	(lasx_xvmaddwev_q_du_d): Remove.
	(lasx_xvmaddwod_h_bu_b): Remove.
	(lasx_xvmaddwod_w_hu_h): Remove.
	(lasx_xvmaddwod_d_wu_w): Remove.
	(lasx_xvmaddwod_q_du_d): Remove.
	* config/loongarch/lsx.md (UNSPEC_LSX_VMADDWEV): Remove.
	(UNSPEC_LSX_VMADDWEV2): Remove.
	(UNSPEC_LSX_VMADDWEV3): Remove.
	(UNSPEC_LSX_VMADDWOD): Remove.
	(UNSPEC_LSX_VMADDWOD2): Remove.
	(UNSPEC_LSX_VMADDWOD3): Remove.
	(lsx_vmaddwev_h_b<u>): Remove.
	(lsx_vmaddwev_w_h<u>): Remove.
	(lsx_vmaddwev_d_w<u>): Remove.
	(lsx_vmaddwev_q_d): Remove.
	(lsx_vmaddwod_h_b<u>): Remove.
	(lsx_vmaddwod_w_h<u>): Remove.
	(lsx_vmaddwod_d_w<u>): Remove.
	(lsx_vmaddwod_q_d): Remove.
	(lsx_vmaddwev_q_du): Remove.
	(lsx_vmaddwod_q_du): Remove.
	(lsx_vmaddwev_h_bu_b): Remove.
	(lsx_vmaddwev_w_hu_h): Remove.
	(lsx_vmaddwev_d_wu_w): Remove.
	(lsx_vmaddwev_q_du_d): Remove.
	(lsx_vmaddwod_h_bu_b): Remove.
	(lsx_vmaddwod_w_hu_h): Remove.
	(lsx_vmaddwod_d_wu_w): Remove.
	(lsx_vmaddwod_q_du_d): Remove.
	* config/loongarch/simd.md (simd_maddw_evod_<mode>_<su>):
	New define_insn.
	(<simd_isa>_<x>vmaddw<ev_od>_<simdfmt_w>_<simdfmt><u>): New
	define_expand.
	(simd_maddw_evod_<mode>_hetero): New define_insn.
	(<simd_isa>_<x>vmaddw<ev_od>_<simdfmt_w>_<simdfmt>u_<simdfmt>):
	New define_expand.
	(<simd_isa>_maddw<ev_od>_q_d<u>_punned): New define_expand.
	(<simd_isa>_maddw<ev_od>_q_du_d_punned): New define_expand.
	* config/loongarch/loongarch-builtins.cc
	(CODE_FOR_lsx_vmaddwev_q_d): Define as a macro to override it
	with the punned expand.
	(CODE_FOR_lsx_vmaddwev_q_du): Likewise.
	(CODE_FOR_lsx_vmaddwev_q_du_d): Likewise.
	(CODE_FOR_lsx_vmaddwod_q_d): Likewise.
	(CODE_FOR_lsx_vmaddwod_q_du): Likewise.
	(CODE_FOR_lsx_vmaddwod_q_du_d): Likewise.
	(CODE_FOR_lasx_xvmaddwev_q_d): Likewise.
	(CODE_FOR_lasx_xvmaddwev_q_du): Likewise.
	(CODE_FOR_lasx_xvmaddwev_q_du_d): Likewise.
	(CODE_FOR_lasx_xvmaddwod_q_d): Likewise.
	(CODE_FOR_lasx_xvmaddwod_q_du): Likewise.
	(CODE_FOR_lasx_xvmaddwod_q_du_d): Likewise.