[cpp20] Fixes undefined behaviour on mid tier regalloc

Fixes undefined behavior, which manifests as a compiler error in C++20 in Google3.

This was caused by using members of vector<T> before T has been defined.
This change just massages the code a bit to get everything in the proper order.

See cl/468678068 on Google3.

Bug: chromium:1284275
Change-Id: I0b65e7f850e8dd1ed482be1b5cc0b8d9d77776eb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3859343
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82745}