std::atomic loads are marked as nodiscard on MSVC. Fix the warning by
feeding the load into the USE() macro.

Bug: chromium:1056170
Change-Id: I72ca42d42d268c4b961d96618250229a53709472
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2120543Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66866}