[sledge] Add Exp.size_of

Summary:
Convenience wrapper of Typ.prim_bit_size_of to construct expressions
denoting the byte-size of sized types.

Reviewed By: mbouaziz

Differential Revision: D14481987

fbshipit-source-id: 44e2c4041

sledge/src/llair/exp.ml

@@ -1227,6 +1227,11 @@ let struct_rec key =
 let convert ?(signed = false) ~dst ~src exp =
   app1 (Convert {signed; dst; src}) exp
 
+let size_of t =
+  Option.bind (Typ.prim_bit_size_of t) ~f:(fun n ->
+      if n % 8 = 0 then Some (integer (Z.of_int (n / 8)) Typ.siz) else None
+  )
+
 (** Transform *)
 
 let map e ~f =

sledge/src/llair/exp.mli

@@ -186,6 +186,7 @@ val struct_rec :
     stack overflow. *)
 
 val convert : ?signed:bool -> dst:Typ.t -> src:Typ.t -> t -> t
+val size_of : Typ.t -> t option
 
 (** Access *)