enhance merge_vars (#4152)

lib/compress.js

@@ -4538,9 +4538,7 @@ merge(Compressor.prototype, {
             if (def.id in references) {
                 var refs = references[def.id];
                 if (!refs) return;
-                if (write ? refs.start !== segment : refs.start.block !== segment.block) {
-                    return references[def.id] = false;
-                }
+                if (refs.start.block !== segment.block) return references[def.id] = false;
                 refs.push(sym);
                 refs.end = segment;
                 if (def.id in prev) {

test/compress/merge_vars.js

@@ -2800,3 +2800,25 @@ lambda_reuse: {
         "string",
     ]
 }
+
+conditional_write: {
+    options = {
+        merge_vars: true,
+        toplevel: true,
+    }
+    input: {
+        var a = "FAIL", b;
+        if (console)
+            a = "PASS";
+        b = [a, 42].join();
+        console.log(b);
+    }
+    expect: {
+        var b = "FAIL", b;
+        if (console)
+            b = "PASS";
+        b = [b, 42].join();
+        console.log(b);
+    }
+    expect_stdout: "PASS,42"
+}