Add test for assembly errors, to verify fix.
Make sure invalid instruction errors are printed just once
(was printing them once per span iteration, so typically twice).

Fixes #13282.

Change-Id: Id5f66f80a80b3bc4832e00084b0a91f1afec7f8f
Reviewed-on: https://go-review.googlesource.com/18858Reviewed-by: Rob Pike <r@golang.org>