Instead of silently truncating integers to their expected range, check
that they're within range and emit errors if not.  Intended to help
narrow down the cause of issue #11617.

Change-Id: Ia7b577270f8438ca7479262702371e26277f1ea7
Reviewed-on: https://go-review.googlesource.com/12050Reviewed-by: Ian Lance Taylor <iant@golang.org>