cmd/vet: allow ^& uintptr arithmetic

cmd/vet: allow ^& uintptr arithmetic

The unsafe.Pointer check allows adding to
and subtracting from uintptrs in order to do
arithmetic.

Some code needs to round uintptrs.
Allow &^ for that purpose.

Updates #11041

Change-Id: Ib90dd2954bb6c78427058271e13f2ce4c4af38fb
Reviewed-on: https://go-review.googlesource.com/27156
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>