enhance assignments & unused (#3428)

closes #3427

test/compress/assignment.js

@@ -311,3 +311,19 @@ issue_3375: {
     }
     expect_stdout: "string"
 }
+
+issue_3427: {
+    options = {
+        assignments: true,
+        sequences: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        (function() {
+            var a;
+            a || (a = {});
+        })();
+    }
+    expect: {}
+}

test/compress/drop-unused.js

@@ -2028,3 +2028,37 @@ issue_3375: {
     }
     expect_stdout: "0 0"
 }
+
+issue_3427_1: {
+    options = {
+        sequences: true,
+        side_effects: true,
+        unused: true,
+    }
+    input: {
+        (function() {
+            var a;
+            a = a || {};
+        })();
+    }
+    expect: {}
+}
+
+issue_3427_2: {
+    options = {
+        unused: true,
+    }
+    input: {
+        (function() {
+            var s = "PASS";
+            console.log(s = s || "FAIL");
+        })();
+    }
+    expect: {
+        (function() {
+            var s = "PASS";
+            console.log(s = s || "FAIL");
+        })();
+    }
+    expect_stdout: "PASS"
+}

test/compress/pure_getters.js

@@ -1187,3 +1187,23 @@ drop_arguments: {
     }
     expect_stdout: "PASS"
 }
+
+issue_3427: {
+    options = {
+        assignments: true,
+        collapse_vars: true,
+        inline: true,
+        pure_getters: "strict",
+        sequences: true,
+        side_effects: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        (function(b) {
+            b.p = 42;
+        })(a || (a = {}));
+    }
+    expect: {}
+}