cmd/compile/internal/types2: minor cleanup of instantiation
This is a clean port of CL 349429 from go/types to types2
with minor adjustments for types2 names.
Change-Id: Ie6a39a01f074acb9e6565ffacb34c94666ae9a95
Reviewed-on: https://go-review.googlesource.com/c/go/+/349999
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>