RISC-V: Fix 'REQUIREMENT' for machine_mode 'MODE' in vector-iterators.md.

gcc/ChangeLog:

	* config/riscv/vector-iterators.md:
	Fix 'REQUIREMENT' for machine_mode 'MODE'.
	* config/riscv/vector.md (@pred_indexed_<order>store<VNX16_QHS:mode>
	<VNX16_QHSI:mode>): change VNX16_QHSI to VNX16_QHSDI.
	(@pred_indexed_<order>store<VNX16_QHS:mode><VNX16_QHSDI:mode>): Ditto.