bpo-40791: Make compare_digest more constant-time. (GH-20444)

* bpo-40791: Make compare_digest more constant-time.

The existing volatile `left`/`right` pointers guarantee that the reads will all occur, but does not guarantee that they will be _used_. So a compiler can still short-circuit the loop, saving e.g. the overhead of doing the xors and especially the overhead of the data dependency between `result` and the reads. That would change performance depending on where the first unequal byte occurs. This change removes that optimization.

(This is change GH-1 from https://bugs.python.org/issue40791

 .)
(cherry picked from commit 31729366)

Co-authored-by: Devin Jeanpierre <jeanpierreda@google.com>