[dev.unified] cmd/compile: extract rtype code from walk

[dev.unified] cmd/compile: extract rtype code from walk

This CL removes (almost*) all reflectdata.{TypePtr,ITabAddr} calls
from package walk. This will allow us to next start adding RType/ITab
fields to IR nodes directly, and have the helpers start returning them
when available instead.

The one survining ITabAddr call is due to ODOTTYPE{,2}, but we already
have ODYNAMICDOTTYPE{,2}, which I plan to have Unified IR always
use. (Longer term, once the Go 1.18 frontend is gone, we can get rid
of ODOTTYPE*, and rename ODYNAMICDOTTYPE*.)

Passes toolstash -cmp.

Change-Id: I5e00da06a93d069abf383d7628e692dd7fd2a1c7
Reviewed-on: https://go-review.googlesource.com/c/go/+/413356
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>