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

arm: 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/arm/iterators.md (v_double_width): New, matching
	V_double_width.
	* config/arm/neon.md (widen_ssum<mode>3): Renamed ...
	(widen_ssum<v_double_width><mode>3, widen_ssum<V_widen_l><mode>3): ...
	into these.
	(widen_usum<mode>3): Renamed ...
	(widen_usum<v_double_width><mode>3, widen_usum<V_widen_l><mode>3): ...
	into these.