The dominator tree is usually computed as part of scheduling (in
{Scheduler::ComputeSchedule}). For tests it was missing, leading to
DCHECK errors in the mid-tier register allocator, which uses the
dominator tree.

R=mslekova@chromium.org

Bug: v8:12330
Change-Id: I02bc8dee3aecb6a1613fa1d07d3aae85cd28de17
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3245114Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77543}