LoongArch: Remove unreachable codes.

gcc/ChangeLog:

	* config/loongarch/loongarch.cc
	(loongarch_split_move): Delete.
	(loongarch_hard_regno_mode_ok_uncached): Likewise.
	* config/loongarch/loongarch.md
	(move_doubleword_fpr<mode>): Likewise.
	(load_low<mode>): Likewise.
	(load_high<mode>): Likewise.
	(store_word<mode>): Likewise.
	(movgr2frh<mode>): Likewise.
	(movfrh2gr<mode>): Likewise.