LoongArch: Optimize additions with immediates

LoongArch: Optimize additions with immediates

1. Use addu16i.d for TARGET_64BIT and suitable immediates.
2. Split one addition with immediate into two addu16i.d or addi.{d/w}
   instructions if possible.  This can avoid using a temp register w/o
   increase the count of instructions.

Inspired by https://reviews.llvm.org/D143710 and
https://reviews.llvm.org/D147222.

Bootstrapped and regtested on loongarch64-linux-gnu.  Ok for GCC 14?

gcc/ChangeLog:

	* config/loongarch/loongarch-protos.h
	(loongarch_addu16i_imm12_operand_p): New function prototype.
	(loongarch_split_plus_constant): Likewise.
	* config/loongarch/loongarch.cc
	(loongarch_addu16i_imm12_operand_p): New function.
	(loongarch_split_plus_constant): Likewise.
	* config/loongarch/loongarch.h (ADDU16I_OPERAND): New macro.
	(DUAL_IMM12_OPERAND): Likewise.
	(DUAL_ADDU16I_OPERAND): Likewise.
	* config/loongarch/constraints.md (La, Lb, Lc, Ld, Le): New
	constraint.
	* config/loongarch/predicates.md (const_dual_imm12_operand): New
	predicate.
	(const_addu16i_operand): Likewise.
	(const_addu16i_imm12_di_operand): Likewise.
	(const_addu16i_imm12_si_operand): Likewise.
	(plus_di_operand): Likewise.
	(plus_si_operand): Likewise.
	(plus_si_extend_operand): Likewise.
	* config/loongarch/loongarch.md (add<mode>3): Convert to
	define_insn_and_split.  Use plus_<mode>_operand predicate
	instead of arith_operand.  Add alternatives for La, Lb, Lc, Ld,
	and Le constraints.
	(*addsi3_extended): Convert to define_insn_and_split.  Use
	plus_si_extend_operand instead of arith_operand.  Add
	alternatives for La and Le alternatives.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/add-const.c: New test.
	* gcc.target/loongarch/stack-check-cfa-1.c: Adjust for stack
	frame size change.
	* gcc.target/loongarch/stack-check-cfa-2.c: Likewise.