[dev.unified] cmd/compile: start setting RType fields for Unified IR

[dev.unified] cmd/compile: start setting RType fields for Unified IR

This CL switches the GOEXPERIMENT=unified frontend to set RType fields
in the simpler cases, and to make it fatal if they're missing.

Subsequent CLs will handle the remaining more complex cases (e.g.,
expressions from later desugaring, and implicit conversions to
interface type).

Change-Id: If6257dcb3916905afd9b8371ea64b85f108ebbfb
Reviewed-on: https://go-review.googlesource.com/c/go/+/413359
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>