[analyzer] semi propagate null values

We don't want to actually move them within the AST as that would likely cause problems with native compilation in may cases. We can however keep track of them and use them for data flow analysis, so something like `var x = null; if ("foo" == x) { }` can get optimized.

src/optimization/analyzer.ml

@@ -348,6 +348,7 @@ module ConstPropagation = DataFlow(struct
 	type t =
 		| Top
 		| Bottom
+		| Null of Type.t
 		| Const of tconstant
 		| EnumValue of int * t list
 
@@ -365,22 +366,26 @@ module ConstPropagation = DataFlow(struct
 	let equals lat1 lat2 = match lat1,lat2 with
 		| Top,Top | Bottom,Bottom -> true
 		| Const ct1,Const ct2 -> ct1 = ct2
+		| Null t1,Null t2 -> t1 == t2
 		| EnumValue(i1,_),EnumValue(i2,_) -> i1 = i2
 		| _ -> false
 
 	let transfer ctx bb e =
 		let rec eval bb e =
 			let wrap = function
 				| Const ct -> mk (TConst ct) t_dynamic null_pos
+				| Null t -> mk (TConst TNull) t e.epos
 				| _ -> raise Exit
 			in
 			let unwrap e = match e.eexpr with
 				| TConst ct -> Const ct
 				| _ -> raise Exit
 			in
 			match e.eexpr with
-			| TConst (TSuper | TThis | TNull) ->
+			| TConst (TSuper | TThis) ->
 				Bottom
+			| TConst TNull ->
+				Null e.etype
 			| TConst ct ->
 				Const ct
 			| TLocal v ->
@@ -465,7 +470,7 @@ module ConstPropagation = DataFlow(struct
 
 	let commit ctx =
 		let inline e i = match get_cell i with
-			| Top | Bottom | EnumValue _ ->
+			| Top | Bottom | EnumValue _ | Null _ ->
 				raise Not_found
 			| Const ct ->
 				let e' = Codegen.type_constant ctx.com (tconst_to_const ct) e.epos in