enhance conditionals (#3798)

test/compress/conditionals.js

@@ -1384,7 +1384,7 @@ hoist_decl: {
     }
     expect: {
         var a, b;
-        x() ? y() : z();
+        (x() ? y : z)();
     }
 }
 

test/compress/drop-unused.js

@@ -1173,6 +1173,7 @@ var_catch_toplevel: {
                 x();
             } catch (a) {
                 var a;
+                var a;
             }
         }();
     }

test/compress/issue-1639.js

@@ -22,8 +22,10 @@ issue_1639_1: {
         console.log(a, b);
     }
     expect: {
-        for (var a = 100, b = 10, L1 = 5; --L1 > 0;)
-            if (--b, 0) var ignore = 0;
+        for (var a = 100, b = 10, L1 = 5; --L1 > 0;) {
+            var ignore;
+            --b;
+        }
         console.log(a, b);
     }
     expect_stdout: "100 6"