IsEqual: Fix behaviour when instantiated with never and unknown

[taiyakihitotsu]

This issue had already existed I think, and I missed it in a previous PR #1231.
Sorry about that.

type test_IsEqual = _IsEqual<never, unknown> // true

[som-sm]

Fails if the order the reversed, i.e., IsEqual<unknown, never> returns true (not false).

[taiyakihitotsu]

@som-sm

Thanks, I fixed it.

(Using [a & b, a | b] to determine which type is unknown and which is never failed in some "intersection with infer" test cases, so I wrote it in a more direct way.)