fix corner case in switch (#1765)

test/compress/issue-1750.js

@@ -24,3 +24,31 @@ case_1: {
     }
     expect_stdout: "0 2"
 }
+
+case_2: {
+    options = {
+        dead_code: true,
+        evaluate: true,
+        switches: true,
+    }
+    input: {
+        var a = 0, b = 1;
+        switch (0) {
+          default:
+            b = 2;
+          case a:
+            a = 3;
+          case 0:
+        }
+        console.log(a, b);
+    }
+    expect: {
+        var a = 0, b = 1;
+        switch (0) {
+          case a:
+            a = 3;
+        }
+        console.log(a, b);
+    }
+    expect_stdout: "3 1"
+}