cmd/compile: more String methods for prove types

These aid in debugging.

Change-Id: Ieb38c996765f780f6103f8c3292639d408c25123
Reviewed-on: https://go-review.googlesource.com/87476


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>