cmd/compile: improve error for wrong type in switch

Fixes #10561.

Provides a better diagnostic message for failed type switch
satisfaction in the case that a value receiver is being used
in place of the pointer receiver that implements and satisfies
the interface.

Change-Id: If8c13ba13f2a8d81bf44bac7c3a66c12921ba921
Reviewed-on: https://go-review.googlesource.com/35235


Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>