[dev.unified] cmd/compile/internal/noder: wire RTTI for implicit conversions

[dev.unified] cmd/compile/internal/noder: wire RTTI for implicit conversions

This CL updates Unified IR to set the TypeWord and SrcRType fields on
interface conversions, which will be necessary for dictionary support
shortly.

Change-Id: I9486b417f514ba4ec2ee8036194aa9ae3ad0ad93
Reviewed-on: https://go-review.googlesource.com/c/go/+/415575
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>