go/internal/gcimporter,cmd/compile: minor clean-up in iimport.go

Make two superficial fixes to iimport.go: rename instType to
instanceType (suggested in CL 349949), and fix a stale comment.

Done in both go/internal/gcimporter and cmd/compile/internal/importer.

Change-Id: Idfdda11a59b036a35824bbb1c101cba3652aeff4
Reviewed-on: https://go-review.googlesource.com/c/go/+/350031


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>