cmd/compile: delete gdata

cmd/compile: delete gdata

All callers to gdata knew the kind of node they were working with,
so all calls to gdata have been replaced with more specific calls.

Some OADDR nodes were constructed solely for the purpose of
passing them to gdata for unwrapping. In those cases, we can now
cut to the chase.

Passes toolstash-check.

Change-Id: Iacc1abefd7f748cb269661a03768d3367319b0b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/228888
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>