compiler: open code some type assertions

Now that type equality is just simple pointer equality, we can
open code some type assertions instead of making runtime calls.

Change-Id: I6acb1e0b1e94059089cb617688e4e4cb26804e18
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/182977
Reviewed-by: Ian Lance Taylor <iant@golang.org>
6 files changed