improve symbol replacement heuristic (#2851)

lib/compress.js

@@ -5237,7 +5237,7 @@ merge(Compressor.prototype, {
                     var name_length = d.name.length;
                     var overhead = 0;
                     if (compressor.option("unused") && !compressor.exposed(d)) {
-                        overhead = (name_length + 2 + value_length) / d.references.length;
+                        overhead = (name_length + 2 + value_length) / (d.references.length - d.assignments);
                     }
                     d.should_replace = value_length <= name_length + overhead ? fn : false;
                 } else {

test/compress/reduce_vars.js

@@ -5435,3 +5435,35 @@ lvalues_def_2: {
     }
     expect_stdout: "2 NaN"
 }
+
+chained_assignments: {
+    options = {
+        evaluate: true,
+        inline: true,
+        reduce_vars: true,
+        sequences: true,
+        side_effects: true,
+        toplevel: true,
+        unsafe: true,
+        unused: true,
+    }
+    input: {
+        function f() {
+            var a = [0x5e, 0xad, 0xbe, 0xef];
+            var b = 0;
+            b |= a[0];
+            b <<= 8;
+            b |= a[1];
+            b <<= 8;
+            b |= a[2];
+            b <<= 8;
+            b |= a[3];
+            return b;
+        }
+        console.log(f().toString(16));
+    }
+    expect: {
+        console.log("5eadbeef");
+    }
+    expect_stdout: "5eadbeef"
+}