cmd/compile/internal/types: replace Type.Val with Type.Elem

This reduces the API surface of Type slightly (for #25056), but also
makes it more consistent with the reflect and go/types APIs.

Passes toolstash-check.

Change-Id: Ief9a8eb461ae6e88895f347e2a1b7b8a62423222
Reviewed-on: https://go-review.googlesource.com/109138


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>