fix collapse_vars on switch (#2578)

lib/compress.js

@@ -873,14 +873,12 @@ merge(Compressor.prototype, {
                     node.expression = node.expression.transform(scanner);
                     for (var i = 0, len = node.body.length; !abort && i < len; i++) {
                         var branch = node.body[i];
-                        if (branch instanceof AST_Case)
+                        if (branch instanceof AST_Case) {
                             branch.expression = branch.expression.transform(scanner);
+                            if (side_effects || !replace_all) break;
+                        }
                     }
-                    for (i = 0; !abort && i < len; i++) {
+                    abort = true;
-                        var branch = node.body[i];
-                        for (var j = 0, len2 = branch.body.length; j < len2; j++)
-                            branch.body[j] = branch.body[j].transform(scanner);
-                    }
                     return node;
                 }
                 // Skip nodes before `candidate` as quickly as possible

test/compress/collapse_vars.js

@@ -1749,7 +1749,7 @@ for_init: {
     }
 }
 
-switch_case: {
+switch_case_1: {
     options = {
         collapse_vars: true,
         unused: true,
@@ -1777,6 +1777,62 @@ switch_case: {
     }
 }
 
+switch_case_2: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = 1, b = 2;
+        switch (b++) {
+          case b:
+            var c = a;
+            var a;
+            break;
+        }
+        console.log(a);
+    }
+    expect: {
+        var a = 1, b = 2;
+        switch (b++) {
+          case b:
+            var c = a;
+            var a;
+            break;
+        }
+        console.log(a);
+    }
+    expect_stdout: "1"
+}
+
+switch_case_3: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var a = 1, b = 2;
+        switch (a) {
+          case a:
+            var b;
+            break;
+          case b:
+            break;
+        }
+        console.log(b);
+    }
+    expect: {
+        var a = 1, b = 2;
+        switch (a) {
+          case a:
+            var b;
+            break;
+          case b:
+            break;
+        }
+        console.log(b);
+    }
+    expect_stdout: "2"
+}
+
 issue_27: {
     options = {
         collapse_vars: true,