fix corner case in collapse_vars (#3574)

fixes #3573

test/compress/collapse_vars.js

@@ -6435,3 +6435,36 @@ call_assign_order: {
     }
     expect_stdout: "PASS PASS"
 }
+
+issue_3573: {
+    options = {
+        collapse_vars: true,
+    }
+    input: {
+        var c = 0;
+        (function(b) {
+            while (--b) {
+                b = NaN;
+                switch (0 / this < 0) {
+                  case c++, false:
+                  case c++, NaN:
+                }
+            }
+        })(3);
+        console.log(c);
+    }
+    expect: {
+        var c = 0;
+        (function(b) {
+            while (--b) {
+                b = NaN;
+                switch (0 / this < 0) {
+                  case c++, false:
+                  case c++, NaN:
+                }
+            }
+        })(3);
+        console.log(c);
+    }
+    expect_stdout: "1"
+}