Move support for negate_iife in the compressor, rather than code generator

(the code generator doesn't maintain enough context to know whether
the return value is important or discarded)

Fixes #272

test/compress/negate-iife.js

@@ -0,0 +1,76 @@
+negate_iife_1: {
+    options = {
+        negate_iife: true
+    };
+    input: {
+        (function(){ stuff() })();
+    }
+    expect: {
+        !function(){ stuff() }();
+    }
+}
+
+negate_iife_2: {
+    options = {
+        negate_iife: true
+    };
+    input: {
+        (function(){ return {} })().x = 10; // should not transform this one
+    }
+    expect: {
+        (function(){ return {} })().x = 10;
+    }
+}
+
+negate_iife_3: {
+    options = {
+        negate_iife: true,
+    };
+    input: {
+        (function(){ return true })() ? console.log(true) : console.log(false);
+    }
+    expect: {
+        !function(){ return true }() ? console.log(false) : console.log(true);
+    }
+}
+
+negate_iife_3: {
+    options = {
+        negate_iife: true,
+        sequences: true
+    };
+    input: {
+        (function(){ return true })() ? console.log(true) : console.log(false);
+        (function(){
+            console.log("something");
+        })();
+    }
+    expect: {
+        !function(){ return true }() ? console.log(false) : console.log(true), function(){
+            console.log("something");
+        }();
+    }
+}
+
+negate_iife_4: {
+    options = {
+        negate_iife: true,
+        sequences: true,
+        conditionals: true,
+    };
+    input: {
+        if ((function(){ return true })()) {
+            console.log(true);
+        } else {
+            console.log(false);
+        }
+        (function(){
+            console.log("something");
+        })();
+    }
+    expect: {
+        !function(){ return true }() ? console.log(false) : console.log(true), function(){
+            console.log("something");
+        }();
+    }
+}