Keep cases that can skip a side-effect

lib/compress/index.js

@@ -1634,19 +1634,20 @@ def_optimize(AST_Switch, function(self, compressor) {
         if (i < body.length && (!default_branch || body.indexOf(default_branch, i) >= i)) {
             // The default behavior is to do nothing. We can take advantage of that to
             // remove all case expressions that are side-effect free that also do
-            // nothing, since they'll default to doing nothing.
-            let pointer = i;
-            for (let j = pointer; j < body.length; j++) {
+            // nothing, since they'll default to doing nothing. But we can't remove any
+            // case expressions before one that would side-effect, since they may cause
+            // the side-effect to be skipped.
+            for (let j = body.length - 1; j >= i; j--) {
                 let branch = body[j];
                 if (branch === default_branch) {
                     default_branch = null;
-                    continue;
+                    body.pop();
+                } else if (!branch.expression.has_side_effects(compressor)) {
+                    body.pop();
+                } else {
+                    break;
                 }
-                if (!branch.expression.has_side_effects(compressor)) continue;
-                // preserve branches that contain a side-effect
-                body[pointer++] = branch;
             }
-            body.length = pointer;
         }
     }