middle-end: refactor WIDEN_SUM_EXPR into convert optab [PR122069]

middle-end: refactor WIDEN_SUM_EXPR into convert optab [PR122069]

This patch changes the widen_[us]sum optabs into a convert optabs such that
targets and specify more than one conversion.

Following this patch are patches rewriting all targets using this change.

While working on this I noticed that the pattern does miss some cases it
could handle if it tried multiple attempts. e.g. if the promotion is from
qi to si, and the target doesn't have this, it should try hi -> si.

But I'm leaving that for now.

gcc/ChangeLog:

	PR middle-end/122069
	* doc/md.texi (widen_ssum@var{n}@var{m}3, widen_usum@var{n}@var{m}3):
	Update docs.
	* optabs.cc (expand_widen_pattern_expr): Add WIDEN_SUM_EXPR as widening.
	* optabs.def (ssum_widen_optab, usum_widen_optab): Convert from direct
	to a conversion optab.
	* tree-vect-patterns.cc (vect_recog_widen_sum_pattern): Change
	vect_supportable_direct_optab_p into vect_supportable_conv_optab_p.