aarch64: Replace some uses of GET_CODE with RTL predicate macros

gcc/
2021-08-17  Alistair_Lee  <alistair.lee@arm.com>

	* rtl.h (CONST_VECTOR_P): New macro.
	* config/aarch64/aarch64.c (aarch64_get_sve_pred_bits): Use RTL
	code testing macros.
	(aarch64_ptrue_all_mode): Likewise.
	(aarch64_expand_mov_immediate): Likewise.
	(aarch64_const_vec_all_in_range_p): Likewise.
	(aarch64_rtx_costs): Likewise.
	(aarch64_legitimate_constant_p): Likewise.
	(aarch64_simd_valid_immediate): Likewise.
	(aarch64_simd_make_constant): Likewise.
	(aarch64_convert_mult_to_shift): Likewise.
	(aarch64_expand_sve_vec_perm): Likewise.
	(aarch64_vec_fpconst_pow_of_2): Likewise.