design: add 19113-signed-shift-counts
design: add 19113-signed-shift-counts

Updates golang/go#19113.

Change-Id: I840d2d67b09db063a0e2e35d545a13b6cf54e593
Reviewed-on: https://go-review.googlesource.com/c/158657
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

golang/proposal,golang/proposal