Revert "[herd] Rework SVE NEG semantics"

This reverts commit 00e50fb.
Because the correct semantics remains to be reviewed.

herd/AArch64Sem.ml

@@ -2381,39 +2381,30 @@ module Make
         let nelem = predicate_nelem src in
         let psize = predicate_psize src in
         let esize = scalable_esize dst in
-        let>= result =
-          let neg_elem idx pred orig cur_val =
-            let>= last = get_predicate_last pred psize idx in
-            let>= () =
-              let cond = Printf.sprintf "ActiveElem(%s, %d)" (A.pp_reg pg) idx in
-              commit_pred_txt (Some cond) ii in
-            M.choiceT last
-              (let>= v = scalable_getlane cur_val idx esize in
-               M.op Op.Sub V.zero v)
-              (scalable_getlane orig idx esize)
-          in
-          let rec reduce n pred orig op =
-            match n with
-            | 0 -> neg_elem n pred orig op
-            | _ ->
-              let>= v = neg_elem n pred orig op
-              and* cur_val = reduce (n-1) pred orig op in
-              scalable_setlane cur_val n esize v
-          in
-          let>= orig, (src, pred) =
-            read_reg_scalable false dst ii >>|
-            M.data_output_union
-              (read_reg_predicate false pg ii)
-              (fun pred ->
-                 let>= any = get_predicate_any pred psize nelem in
-                 let>* () =
-                   let cond = Printf.sprintf "AnyActive(%s)" (A.pp_reg pg) in
-                   commit_pred_txt (Some cond) ii in
-                 let>= src = M.choiceT any (read_reg_scalable false src ii) (mzero) in
-                 M.unitT (src, pred)) in
-          reduce (nelem-1) pred orig src
-        in
-        write_reg_scalable dst result ii
+        read_reg_scalable false dst ii >>|
+        read_reg_predicate false pg ii >>= fun (orig,pred) ->
+          get_predicate_any pred psize nelem >>= fun any ->
+            M.choiceT
+            any
+            (read_reg_scalable false src ii >>= fun src ->
+              let negate orig cur_val idx =
+                get_predicate_last pred psize idx >>= fun last ->
+                M.choiceT
+                  last
+                  (scalable_getlane cur_val idx esize
+                   >>= M.op Op.Sub V.zero)
+                  (scalable_getlane orig idx esize)
+                >>= fun v ->
+                  scalable_setlane cur_val idx esize v
+              in
+              let rec reduce orig n op =
+              match n with
+              | 0 -> op >>= fun old_val -> negate orig old_val n
+              | _ -> reduce orig (n-1) (op >>= fun old_val -> negate orig old_val n)
+              in
+              reduce orig (nelem-1) (M.unitT src))
+            (M.unitT orig) >>= fun v ->
+              write_reg_scalable dst v ii
 
       let while_op compare unsigned p var r1 r2 ii =
         let psize = predicate_psize p in