rs6000: Adjust altivec 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/rs6000/altivec.md (udot_prod<mode>): Renamed to...
	(udot_prodv4si<mode>): ...this.
	(sdot_prodv8hi): Renamed to...
	(sdot_prodv4siv8hi): ...this.