cmd/compile/internal/types2: optimize common case in structuralType
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 <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,golang/go,golang/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,golang/go