Selector: Use shallow document comparisons in uniqueSort

Selector: Use shallow document comparisons in uniqueSort

IE/Edge sometimes crash when comparing documents between frames using the strict
equality operator (`===` & `!==`). Funnily enough, shallow comparisons
(`==` & `!=`) work without crashing.

The change to shallow comparisons in `src/selector.js` was done in gh-4471 but
relevant changes in `src/selector/uniqueSort.js` were missed. Those changes
have landed in Sizzle in jquery/sizzle#459.

Fixes gh-4441
Closes gh-4512
Ref gh-4471
Ref jquery/sizzle#459