[dev.typeparams] cmd/compile/internal/types2: a type parameter is a valid type case in a type switch
Likewise for type assertions.
This is a port of https://golang.org/cl/273127 to dev.typeparams.
Updates #42758.
Change-Id: If93246371c3555e067b0043f0caefaac99101ebc
Reviewed-on: https://go-review.googlesource.com/c/go/+/273128
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>