Walking the dominator tree can be slow when that tree is very deep,
and since it's typically done at least once for every BasicBlock,
overall cost is approximately quadratic.
With some (sparse) caching, we can get significant speedups for
very little extra memory consumption.
In the specific function I looked at, tree depth was around 11,500,
and this patch speeds up the Scheduling phase from 42 seconds to 0.2
seconds, while increasing its memory consumption from 113.1 to 113.4
megabytes.

Change-Id: Iaa32d249a30f62269858d090fbd8924d16d3a9f4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3218157
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77356}