[dev.unified] cmd/compile/internal/reflectdata: prefer ITabAddrAt in ConvIfaceTypeWord

[dev.unified] cmd/compile/internal/reflectdata: prefer ITabAddrAt in ConvIfaceTypeWord

We already have an explicit `pos` parameter, so we should use
ITabAddrAt instead of ITabAddr (which uses `base.Pos` instead).

Change-Id: I7c8c5ae93d0ae7a6467cc972575cb547981576f0
Reviewed-on: https://go-review.googlesource.com/c/go/+/415578
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>