RISC-V: Fix magic number of RVV auto-vectorization expander

RISC-V: Fix magic number of RVV auto-vectorization expander

This simple patch fixes the magic number, remove magic number make codes
more reasonable.

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (expand_vec_series): Remove magic number.
	(expand_const_vector): Ditto.
	(legitimize_move): Ditto.
	(sew64_scalar_helper): Ditto.
	(expand_tuple_move): Ditto.
	(expand_vector_init_insert_elems): Ditto.
	* config/riscv/riscv.cc (vector_zero_call_used_regs): Ditto.

Signed-off-by: Juzhe-Zhong <juzhe.zhong@rivai.ai>