improve pure_getters (#1786)

- property access to `null` & `undefined` always has side effects
- utilise `reduce_vars` to determine safe property access
- may-be cases treated as side effects unless `unsafe`

test/compress/pure_getters.js

@@ -0,0 +1,85 @@
+side_effects: {
+    options = {
+        pure_getters: true,
+        reduce_vars: false,
+        side_effects: true,
+        toplevel: true,
+        unsafe: false,
+    }
+    input: {
+        var a, b = null, c = {};
+        a.prop;
+        b.prop;
+        c.prop;
+        d.prop;
+        null.prop;
+        (void 0).prop;
+        undefined.prop;
+    }
+    expect: {
+        var a, b = null, c = {};
+        a.prop;
+        b.prop;
+        c.prop;
+        d.prop;
+        null.prop;
+        (void 0).prop;
+        (void 0).prop;
+    }
+}
+
+side_effects_reduce_vars: {
+    options = {
+        pure_getters: true,
+        reduce_vars: true,
+        side_effects: true,
+        toplevel: true,
+        unsafe: false,
+    }
+    input: {
+        var a, b = null, c = {};
+        a.prop;
+        b.prop;
+        c.prop;
+        d.prop;
+        null.prop;
+        (void 0).prop;
+        undefined.prop;
+    }
+    expect: {
+        var a, b = null, c = {};
+        a.prop;
+        b.prop;
+        d.prop;
+        null.prop;
+        (void 0).prop;
+        (void 0).prop;
+    }
+}
+
+side_effects_unsafe: {
+    options = {
+        pure_getters: true,
+        reduce_vars: false,
+        side_effects: true,
+        toplevel: true,
+        unsafe: true,
+    }
+    input: {
+        var a, b = null, c = {};
+        a.prop;
+        b.prop;
+        c.prop;
+        d.prop;
+        null.prop;
+        (void 0).prop;
+        undefined.prop;
+    }
+    expect: {
+        var a, b = null, c = {};
+        d;
+        null.prop;
+        (void 0).prop;
+        (void 0).prop;
+    }
+}