rs6000: convert widen_[us]sum into convert optab [PR122069]

rs6000: convert widen_[us]sum into convert optab [PR122069]

This patch is a mechanical rewrite of the widen_[us]sum optabs from a direct to
a conversion optab.  The result of which requires the output mode to be added to
the existing patterns.

No change in functionality is expected.

gcc/ChangeLog:

	PR middle-end/122069
	* config/rs6000/altivec.md (widen_usum<mode>3): Rename ...
	(widen_usumv4si<mode>3): ... to this.
	(widen_ssumv16qi3): Rename ...
	(widen_ssumv4siv16qi3): ... to this.
	(widen_ssumv8hi3): Rename ...
	(widen_ssumv4siv8hi3): ... to this.