cmd/compile: abstract type type+value obtained from types2

In preparation for encoding it in a more efficient way.

Change-Id: I299dd2befc3d07107a1b7b49225bbb9f2e48a343
Reviewed-on: https://go-review.googlesource.com/c/go/+/432896


Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>