&^ and << have equal precedence.  Add some parentheses to make sure
we shift before we andnot.

Fixes #27829

Change-Id: Iba8576201f0f7c52bf9795aaa75d15d8f9a76811
Reviewed-on: https://go-review.googlesource.com/136899Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>