mips: Adjust dot-product backend patterns

Following the migration of the dot_prod optab from a direct to a
conversion-type optab, ensure all back-end patterns incorporate the
second machine mode into pattern names.

gcc/ChangeLog:

	* config/mips/loongson-mmi.md (sdot_prodv4hi): Renamed to...
	(sdot_prodv2siv4hi): ...this.