LoongArch: Simplify lsx_vpick description

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

This is not suitable for LASX where lasx_xvpick has a different
semantic.

gcc/ChangeLog:

	* config/loongarch/simd.md (LVEC): New define_mode_attr.
	(simdfmt_as_i): Make it same as simdfmt for integer vector
	modes.
	(_f): New define_mode_attr.
	* config/loongarch/lsx.md (lsx_vpickev_b): Remove.
	(lsx_vpickev_h): Remove.
	(lsx_vpickev_w): Remove.
	(lsx_vpickev_w_f): Remove.
	(lsx_vpickod_b): Remove.
	(lsx_vpickod_h): Remove.
	(lsx_vpickod_w): Remove.
	(lsx_vpickev_w_f): Remove.
	(lsx_pick_evod_<mode>): New define_insn.
	(lsx_<x>vpick<ev_od>_<simdfmt_as_i><_f>): New
	define_expand.