The value types in type assertions and type switches cannot be
constraint types (if there are, an error was reported earlier),
so there is no need to check again that they are not constraint
types.
This permits merging the ordinaryType call with varType, which
is the only place where it's needed.
Change-Id: I44a852377b3dddf53692f764e588801fb3d3c0a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/346291
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>