cmd/compile/internal/types2: optimize common case in structuralType
Most of the time we don't have a type parameter. Avoid using a
closure in that case.
While at it, rename argument from typ to t (to match style in
that file), and clarify the doc string.
Change-Id: Ie62821073f60f353526263f8b380bad9f72d842e
Reviewed-on: https://go-review.googlesource.com/c/go/+/363668
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>