[dev.typeparams] cmd/compile/internal/types2: bring over subst.go changes from go/types

This CL make this code match closely with the go/types version
(see https://golang.org/cl/276253).

Change-Id: I2b7841309fdbda7e2c9533768bd98317729b13b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/276814


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