v3_kernel: Stop ensuring one return value in #k_try{}

For unclear reasons, v3_kernel attempts to guarantee that #k_try{}
always has at least one return value, even if it will never be
used. I said "attempts", because the handler block that is executed
when an exception is caught does not have the same guarantee. That
means that if an exception is thrown, the return value will not
actually be set.

In practice, however, this is not a problem for the existing code
generator (v3_codegen). The generated code will still be safe.
If we are to rewrite the code generator to generate an SSA-based
intermediate format, this inconsistency *will* cause problems
when creating phi nodes.

While at it, also remove an unecessary creation of new variables
in generation of #k_try_enter{}.

lib/compiler/src/v3_kernel.erl

@@ -2377,26 +2377,23 @@ uexpr(#k_try{anno=A,arg=A0,vars=Vs,body=B0,evars=Evs,handler=H0},
 	    {A1,Au,St2} = ubody(A0, {break,Avs}, St1),
 	    {B1,Bu,St3} = ubody(B0, Br, St2),
 	    {H1,Hu,St4} = ubody(H0, Br, St3),
-	    {Rs1,St5} = ensure_return_vars(Rs0, St4),
 	    Used = union([Au,subtract(Bu, lit_list_vars(Vs)),
 			  subtract(Hu, lit_list_vars(Evs))]),
-	    {#k_try{anno=#k{us=Used,ns=lit_list_vars(Rs1),a=A},
-		    arg=A1,vars=Vs,body=B1,evars=Evs,handler=H1,ret=Rs1},
-	     Used,St5}
+	    {#k_try{anno=#k{us=Used,ns=lit_list_vars(Rs0),a=A},
+		    arg=A1,vars=Vs,body=B1,evars=Evs,handler=H1,ret=Rs0},
+	     Used,St4}
     end;
 uexpr(#k_try{anno=A,arg=A0,vars=Vs,body=B0,evars=Evs,handler=H0},
       return, St0) ->
     {Avs,St1} = new_vars(length(Vs), St0),	%Need dummy names here
     {A1,Au,St2} = ubody(A0, {break,Avs}, St1),	%Must break to clean up here!
     {B1,Bu,St3} = ubody(B0, return, St2),
     {H1,Hu,St4} = ubody(H0, return, St3),
-    NumNew = 1,
-    {Ns,St5} = new_vars(NumNew, St4),
     Used = union([Au,subtract(Bu, lit_list_vars(Vs)),
 		  subtract(Hu, lit_list_vars(Evs))]),
-    {#k_try_enter{anno=#k{us=Used,ns=Ns,a=A},
+    {#k_try_enter{anno=#k{us=Used,ns=[],a=A},
 		  arg=A1,vars=Vs,body=B1,evars=Evs,handler=H1},
-     Used,St5};
+     Used,St4};
 uexpr(#k_catch{anno=A,body=B0}, {break,Rs0}, St0) ->
     {Rb,St1} = new_var(St0),
     {B1,Bu,St2} = ubody(B0, {break,[Rb]}, St1),