This is an inconsequential consequence of updating
math/big to use math/bits.

Better would be to teach the vet shift test
to size int/uint/uintptr to the platform in use,
eliminating the whole category of "might be too small".
Filed #19321 for that.

Change-Id: I7e0b837bd329132d7a564468c18502dd2e724fc6
Reviewed-on: https://go-review.googlesource.com/37576
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>