improve unused (#1832)

- extract leading value with side-effects out of `var` statement
- reduce scanning of `AST_Definitions` from 3 passes to just once

test/compress/collapse_vars.js

@@ -701,10 +701,10 @@ collapse_vars_lvalues_drop_assign: {
         function f2(x) { var z = x, a = ++z; return z += a; }
         function f3(x) { var a = (x -= 3); return x + a; }
         function f4(x) { var a = (x -= 3); return x + a; }
-        function f5(x) { var v = (e1(), e2()), c = v = --x; return x - c; }
+        function f5(x) { e1(); var v = e2(), c = v = --x; return x - c; }
         function f6(x) { e1(), e2(); return --x - x; }
-        function f7(x) { var c = (e1(), e2() - x); return x - c; }
-        function f8(x) { var v = (e1(), e2()); return x - (v - x); }
+        function f7(x) { e1(); var c = e2() - x; return x - c; }
+        function f8(x) { e1(); var v = e2(); return x - (v - x); }
         function f9(x) { e1(); return e2() - x - x; }
     }
 }

test/compress/drop-unused.js

@@ -935,7 +935,8 @@ issue_1715_3: {
             try {
                 console;
             } catch (a) {
-                var a = x();
+                var a;
+                x();
             }
         }
         f();

test/compress/reduce_vars.js

@@ -53,9 +53,7 @@ reduce_vars: {
             console.log(-3);
             eval("console.log(a);");
         })(eval);
-        (function() {
-            return "yes";
-        })();
+        "yes";
         console.log(2);
     }
     expect_stdout: true
@@ -1699,9 +1697,7 @@ redefine_arguments_2: {
         console.log(function() {
             var arguments;
             return typeof arguments;
-        }(), function() {
-            return"number";
-        }(), function(x) {
+        }(), "number", function(x) {
             var arguments = x;
             return typeof arguments;
         }());
@@ -1810,9 +1806,7 @@ redefine_farg_2: {
         console.log(function(a) {
             var a;
             return typeof a;
-        }([]), function() {
-            return "number";
-        }(),function(a, b) {
+        }([]), "number",function(a, b) {
             var a = b;
             return typeof a;
         }([]));