cmd/compile/internal/types2: range clause to accept type sets with single underlying types

This generalizes range clauses. Removed some dead code and cleaned
up the surrounding bits.

Change-Id: Icd8384205afa3f52b7e7df9abed5de2bb556861d
Reviewed-on: https://go-review.googlesource.com/c/go/+/357778


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