simplify do-while into for (#2907)

fixes #2904

lib/compress.js

@@ -3672,6 +3672,13 @@ merge(Compressor.prototype, {
                 ]
             }).optimize(compressor);
         }
+        if (self.body instanceof AST_SimpleStatement) return make_node(AST_For, self, {
+            condition: make_sequence(self.condition, [
+                self.body.body,
+                self.condition
+            ]),
+            body: make_node(AST_EmptyStatement, self)
+        }).optimize(compressor);
         return self;
     });
 

test/compress/loops.js

@@ -605,3 +605,20 @@ issue_2740_5: {
     }
     expect_stdout: "0 undefined"
 }
+
+issue_2904: {
+    options = {
+        join_vars: true,
+        loops: true,
+    }
+    input: {
+        var a = 1;
+        do {
+            console.log(a);
+        } while (--a);
+    }
+    expect: {
+        for (var a = 1; console.log(a), --a;);
+    }
+    expect_stdout: "1"
+}

test/compress/reduce_vars.js

@@ -1178,9 +1178,7 @@ toplevel_on_loops_1: {
             console.log("bar:", --x);
         }
         var x = 3;
-        do
-            bar();
-        while (x);
+        for (;bar(), x;);
     }
     expect_stdout: true
 }
@@ -1208,9 +1206,7 @@ toplevel_off_loops_1: {
             console.log("bar:", --x);
         }
         var x = 3;
-        do
-            bar();
-        while (x);
+        for (;bar(), x;);
     }
     expect_stdout: true
 }
@@ -1265,9 +1261,7 @@ toplevel_off_loops_2: {
             console.log("bar:");
         }
         var x = 3;
-        do
-            bar();
-        while (x);
+        for (;bar(), x;);
     }
 }