cmd/compile: fix expression switches using type parameters
Both the thing we're switching on, as well as the cases we're switching for.
Convert anything containing a type parameter to interface{} before the
comparison happens.
Fixes #51522
Change-Id: I97ba9429ed332cb7d4240cb60f46d42226dcfa5f
Reviewed-on: https://go-review.googlesource.com/c/go/+/391594
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com>