aarch64: Rename and improve aarch64_split_imm24

aarch64: Rename and improve aarch64_split_imm24

Two of the three uses of aarch64_imm24 included the important follow-up
tests vs aarch64_move_imm and aarch64_plus_operand.  Lack of the exclusion
within aarch64_if_then_else_costs produced incorrect costing.

Since aarch64_split_imm24 has already matched a non-negative CONST_INT,
drill down from aarch64_plus_operand to aarch64_uimm12_shift.

gcc:
	* config/aarch64/predicates.md (aarch64_split_imm24): Rename from
	aarch64_imm24; exclude aarch64_move_imm and aarch64_uimm12_shift.
	* config/aarch64/aarch64.md (*aarch64_bcond_wide_imm<GPI>):
	Update for aarch64_split_imm24.
	(*compare_cstore<GPI>_insn): Likewise.
	* config/aarch64/aarch64.cc (aarch64_if_then_else_costs): Likewise.