[dev.typeparams] cmd/compile/internal/types: review of selection.go

Except for the package name, this file is unchanged from the go/types version.

f=selection.go; diff $f ../../../../go/types/$f
7c7
< package types2
---
> package types

Change-Id: I09c26a744f445ec992c554d293e3ca9896b5c849
Reviewed-on: https://go-review.googlesource.com/c/go/+/265698


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>