Add reference equality check to SetEquals methods.

The SetEquals methods are rather expensive, and it's a quick and super-effective check to compare reference equality between the two collections before engaging in the expensive work.
This doesn't test reference equality. I just realized in the creation of this change that we didn't have any set equality tests that took a set as the argument -- they were always just arrays. So this particular additional line causes an equivalent set (not reference equal) to be tested as an argument.