[dev.unified] cmd/compile: write iface conversion RTTI into unified IR
This CL changes convRTTI into a serialization method too, like the
previous CL's rtype method. And again, currently this just builds on
the existing type serialization logic, but will eventually be changed
to use dictionary lookups where appropriate.
Change-Id: I551aef8ade24b08dc6206f06ace86d91e665f5c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/419457 Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>