fix incorrect context in variable substitution (#1791)

`AST_Node.optimize()` is context-aware, so don't cache its results to be used elsewhere.

Also fixed a few cases of AST corruption and beef up safety of `pure_getters`.

test/compress/reduce_vars.js

@@ -1866,3 +1866,75 @@ delay_def: {
     }
     expect_stdout: true
 }
+
+booleans: {
+    options = {
+        booleans: true,
+        evaluate: true,
+        reduce_vars: true,
+    }
+    input: {
+        console.log(function(a) {
+            if (a != 0);
+            switch (a) {
+              case 0:
+                return "FAIL";
+              case false:
+                return "PASS";
+            }
+        }(false));
+    }
+    expect: {
+        console.log(function(a) {
+            if (!1);
+            switch (!1) {
+              case 0:
+                return "FAIL";
+              case !1:
+                return "PASS";
+            }
+        }(!1));
+    }
+    expect_stdout: "PASS"
+}
+
+side_effects_assign: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        sequences: true,
+        side_effects: true,
+        toplevel: true,
+    }
+    input: {
+        var a = typeof void (a && a.in == 1, 0);
+        console.log(a);
+    }
+    expect: {
+        var a = typeof void (a && a.in);
+        console.log(a);
+    }
+    expect_stdout: "undefined"
+}
+
+pure_getters: {
+    options = {
+        pure_getters: true,
+        reduce_vars: true,
+        side_effects: true,
+        toplevel: true,
+    }
+    input: {
+        try {
+            var a = (a.b, 2);
+        } catch (e) {}
+        console.log(a);
+    }
+    expect: {
+        try {
+            var a = (a.b, 2);
+        } catch (e) {}
+        console.log(a);
+    }
+    expect_stdout: "undefined"
+}