always keep declarations found in unreachable code

a few more tests and some cleanups.

test/compress/conditionals.js

@@ -0,0 +1,74 @@
+ifs_1: {
+    options = {
+        conditionals: true
+    };
+    input: {
+        if (foo) bar();
+        if (!foo); else bar();
+        if (foo); else bar();
+        if (foo); else;
+    }
+    expect: {
+        foo&&bar();
+        foo&&bar();
+        foo||bar();
+        foo;
+    }
+}
+
+ifs_2: {
+    options = {
+        conditionals: true
+    };
+    input: {
+        if (foo) {
+            x();
+        } else if (bar) {
+            y();
+        } else if (baz) {
+            z();
+        }
+
+        if (foo) {
+            x();
+        } else if (bar) {
+            y();
+        } else if (baz) {
+            z();
+        } else {
+            t();
+        }
+    }
+    expect: {
+        foo ? x() : bar ? y() : baz && z();
+        foo ? x() : bar ? y() : baz ? z() : t();
+    }
+}
+
+ifs_3_should_warn: {
+    options = {
+        conditionals : true,
+        dead_code    : true,
+        evaluate     : true,
+        booleans     : true
+    };
+    input: {
+        if (x && !(x + "1") && y) { // 1
+            var qq;
+            foo();
+        } else {
+            bar();
+        }
+
+        if (x || !!(x + "1") || y) { // 2
+            foo();
+        } else {
+            var jj;
+            bar();
+        }
+    }
+    expect: {
+        var qq; bar();          // 1
+        var jj; foo();          // 2
+    }
+}