compiler: open code string equality

Open code string equality with builtin memcmp. This allows
further optimizations in the backend.

Change-Id: I09618972d46fd09b0b4acee148e56a671f69d7d2
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/183538
Reviewed-by: Ian Lance Taylor <iant@golang.org>