The register state is accessed a lot in the mid-tier register allocator.
Instead of going through an accessor with a DCHECK, just access
directly. This makes stepping for debugging a lot easier, and will
result in an easy-to-debug nullptr access if the register state is not
initialized.

R=thibaudm@chromium.org

Bug: v8:12330
Change-Id: Icf4d1cc187a34f28ee44fc9b80ee5d765aa14b9a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3372911Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78516}