See also Turbofan's JSContextSpecialization reducer.

For all context loads and stores, this CL implements:

1) depth reduction through graph walks (even without FCS)
2) conversion from the context node to a heap constant
3) if possible, conversion of a load of an immutable context slot load
   to a heap constant

Bug: v8:7700
Change-Id: Ie4d1acd0ff206f25dd5373a860d23b006a31dcee
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3904914Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Linke <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83418}