When using if, the decompression_elimination goes out of scope and we need
it alive. We were using #if guards before but it was changed in
https://chromium-review.googlesource.com/c/v8/v8/+/1815134 to use
FLAG_turbo_decompression_elimination as well.

It was breaking a clusterfuzz builder before this CL:
https://ci.chromium.org/p/v8/builders/ci/V8%20Clusterfuzz%20Linux64%20ASAN%20arm64%20-%20debug%20builder/11562

We can pull the variable out of the if, and then have an USE for the case that it
doesn't get added (i.e non pointer compression).

Bug: v8:7703
Change-Id: Ia0a9f50ebf2cbb41b60e3fd1110d7c7e7eca365d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1849528Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64211}