PCScan: Avoid subs while checking super-pages.

Instead, bitmask by giga-cage offset.

Bug: 11297512
Change-Id: If4a431c2361243818d8f63b40e3190ed7ff88092
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2658771


Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#849098}