RISC-V: Use riscv_2x_xlen_mode_p [NFC]

RISC-V: Use riscv_2x_xlen_mode_p [NFC]

Use riscv_v_ext_mode_p to check the mode size is 2x XLEN, instead of
using "(GET_MODE_UNIT_SIZE (mode) == (UNITS_PER_WORD * 2))".

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_legitimize_move): Use
	riscv_2x_xlen_mode_p.
	(riscv_binary_cost): Ditto.
	(riscv_hard_regno_mode_ok): Ditto.