The condition to detect an inline backing store was wrong and we would
try to access the heap-allocated store even for inline stores.

Drive-by: Use kBitsPerSystemPointer and the new
kBitsPerSystemPointerLog2 constants.

Change-Id: I19d0245ae82642a788c967534ab2a84464d56a67
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1890093
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64667}