output optimal representations of NaN & Infinity (#1723)

- move these optimisations out from `Compressor` to `OutputStream`
- fixes behaviour inconsistency when running uglified code from global or module levels due to redefinition

test/compress/conditionals.js

@@ -840,8 +840,8 @@ equality_conditionals_false: {
         f(0, true, 0),
         f(1, 2, 3),
         f(1, null, 3),
-        f(NaN),
-        f(NaN, "foo");
+        f(0/0),
+        f(0/0, "foo");
     }
     expect_stdout: true
 }
@@ -888,8 +888,8 @@ equality_conditionals_true: {
         f(0, true, 0),
         f(1, 2, 3),
         f(1, null, 3),
-        f(NaN),
-        f(NaN, "foo");
+        f(0/0),
+        f(0/0, "foo");
     }
     expect_stdout: true
 }