RISC-V: Eliminate the magic number in riscv-v.cc

RISC-V: Eliminate the magic number in riscv-v.cc

This patch would like to remove the magic number in the riscv-v.cc, and
align the same value to one macro.

Signed-off-by: Pan Li <pan2.li@intel.com>
gcc/ChangeLog:

	* config/riscv/riscv-v.cc (emit_vlmax_insn): Eliminate the
	magic number.
	(emit_nonvlmax_insn): Ditto.
	(emit_vlmax_merge_insn): Ditto.
	(emit_vlmax_cmp_insn): Ditto.
	(emit_vlmax_cmp_mu_insn): Ditto.
	(expand_vec_series): Ditto.

Signed-off-by: Pan Li <pan2.li@intel.com>