How about this?

Not sure about the DEF_INTERNAL documentation I rewrote in 
internal-fn.def, was struggling to word these, so improvements welcome!

gcc/ChangeLog:

2023-04-25  Andre Vieira  <andre.simoesdiasvieira@arm.com>
             Joel Hutton  <joel.hutton@arm.com>
             Tamar Christina  <tamar.christina@arm.com>

         * config/aarch64/aarch64-simd.md 
(vec_widen_<su>addl_lo_<mode>): Rename
         this ...
         (vec_widen_<su>add_lo_<mode>): ... to this.
         (vec_widen_<su>addl_hi_<mode>): Rename this ...
         (vec_widen_<su>add_hi_<mode>): ... to this.
         (vec_widen_<su>subl_lo_<mode>): Rename this ...
         (vec_widen_<su>sub_lo_<mode>): ... to this.
         (vec_widen_<su>subl_hi_<mode>): Rename this ...
         (vec_widen_<su>sub_hi_<mode>): ...to this.
         * doc/generic.texi: Document new IFN codes.
	* internal-fn.cc (ifn_cmp): Function to compare ifn's for 
sorting/searching.
	(lookup_hilo_internal_fn): Add lookup function.
	(commutative_binary_fn_p): Add widen_plus fn's.
	(widening_fn_p): New function.
	(narrowing_fn_p): New function.
         (direct_internal_fn_optab): Change visibility.
	* internal-fn.def (DEF_INTERNAL_WIDENING_OPTAB_FN): Macro to define an
         internal_fn that expands into multiple internal_fns for widening.
         (DEF_INTERNAL_NARROWING_OPTAB_FN): Likewise but for narrowing.
         (IFN_VEC_WIDEN_PLUS, IFN_VEC_WIDEN_PLUS_HI, IFN_VEC_WIDEN_PLUS_LO,
          IFN_VEC_WIDEN_PLUS_EVEN, IFN_VEC_WIDEN_PLUS_ODD,
          IFN_VEC_WIDEN_MINUS, IFN_VEC_WIDEN_MINUS_HI, 
IFN_VEC_WIDEN_MINUS_LO,
          IFN_VEC_WIDEN_MINUS_ODD, IFN_VEC_WIDEN_MINUS_EVEN): Define 
widening
         plus,minus functions.
	* internal-fn.h (direct_internal_fn_optab): Declare new prototype.
	(lookup_hilo_internal_fn): Likewise.
	(widening_fn_p): Likewise.
	(Narrowing_fn_p): Likewise.
	* optabs.cc (commutative_optab_p): Add widening plus optabs.
	* optabs.def (OPTAB_D): Define widen add, sub optabs.
         * tree-cfg.cc (verify_gimple_call): Add checks for widening ifns.
         * tree-inline.cc (estimate_num_insns): Return same
         cost for widen add and sub IFNs as previous tree_codes.
	* tree-vect-patterns.cc (vect_recog_widen_op_pattern): Support
         patterns with a hi/lo or even/odd split.
         (vect_recog_sad_pattern): Refactor to use new IFN codes.
         (vect_recog_widen_plus_pattern): Likewise.
         (vect_recog_widen_minus_pattern): Likewise.
         (vect_recog_average_pattern): Likewise.
	* tree-vect-stmts.cc (vectorizable_conversion): Add support for
         _HILO IFNs.
	(supportable_widening_operation): Likewise.
         * tree.def (WIDEN_SUM_EXPR): Update example to use new IFNs.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/vect-widen-add.c: Test that new
     IFN_VEC_WIDEN_PLUS is being used.
	* gcc.target/aarch64/vect-widen-sub.c: Test that new
     IFN_VEC_WIDEN_MINUS is being used.