cmd/compile: remove Addable flag

This flag is supposed to indicate whether the expression is
"addressable"; but in practice, we infer this from other
attributes about the expression (e.g., n.Op and n.Class()).

Passes toolstash-check.

Change-Id: I19352ca07ab5646e232d98e8a7c1c9aec822ddd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/200897


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