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Currently, the "get offset of a pointer within its allocated slot"
functionality, which is used extensively by several MiraclePtr
implementations, performs an expensive modulo operation. This commit
replaces it with a cheaper sequence of multiplications and bit shifts.

Bug: 1092288
Change-Id: I866689a79fff704b5f83fc07d8f28b3386ef644d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2335443
Commit-Queue: Sergei Glazunov <glazunov@google.com>
Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802194}