SI-6968 Simple Tuple patterns aren't irrefutable

Reverts part of c82ecab. The parser can't assume that
a pattern `(a, b)` will match, as results of
`.isInstanceOf[Tuple2]` can't be statically known until
after the typer.

The reopens SI-1336 and SI-5589, in exchange for fixing
this regression SI-6968. Keeping all three fixed will require
some major surgery.

Review by @paulp