SI-4577 singleton type pattern test should use `eq`, not `==`

I find it hard to imagine anyone is relying on `case x: foo.type =>`
erroneously being compiled to `foo == x` instead of the spec'ed `foo eq x`,
so let's finally fix this.
Merge pull request #3558 from adriaanm/t4577

SI-4577 singleton type pattern test should use `eq`, not `==`