[herd]: Optimise SVE predicate regsiters.

The `PTRUE` instruction now transmits stored value
to semantic-time environement.

As a result, predicated instructions may sometimes
decide whether to access or not at semantic-time, leading to
a dramatic decrease of execution candidates number.

herd/AArch64Sem.ml

@@ -2323,7 +2323,7 @@ module Make
           else M.unitT old_val
         in
         repeat V.zero 0 >>= fun (new_val) ->
-        write_reg_predicate p new_val ii
+        write_reg_predicate p new_val ii >>! new_val
 
       let mov_sv r k shift ii =
         let open AArch64Base in
@@ -3265,7 +3265,8 @@ module Make
               M.unitT ())
         | I_PTRUE(p,pattern) ->
           check_sve inst;
-          !(ptrue p pattern ii)
+          ptrue p pattern ii
+          >>= nextSet p
         | I_WHILELT(p,var,r1,r2) ->
           check_sve inst;
           !!(while_op (M.op Op.Lt) false p var r1 r2 ii)