c6x: 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/c6x/c6x.md (sdot_prodv2hi): Renamed to...
	(sdot_prodsiv2hi): ...this.