For functions with a very large stack, the debug side table repeats a
lot of information: Most values will be spilled to the stack, still
every single entry in the debug side table repeats information about
them (type, stack offset). This leads to the size of the debug side
table to be quadratic in the size of the function.

In the linked bug, the generation of the debug side table took ~400ms,
whereas Liftoff compilation alone just took 16ms.

This CL optimized the debug side table by delta-encoding the entries,
i.e. only storing stack slots that changed. This reduces the size of the
table significantly, at the cost of making lookup slower, since that now
has to search the table backwards for the last entry that had
information about a specific slot. For now, this seems like a good
compromise. If it turns out to be a problem, we could speed up the
lookup by either forcing a full dump of the stack state after N entries,
or by dynamically inserting new entries during lookup, whenever we find
that we had to search backwards more than N entries. That would speed up
subsequent lookups then.

On the reproducer in the linked bug, this change reduces the time to
generate the debug side table from ~400ms to ~120ms.
Before this CL, the debug side table has 13,314 entries with a total of
38,599,606 stack value entries. After this CL, it shrinks to 20,037
stack value entries in the 13,314 entries (average of ~1.5 instead of
~2,899).

R=thibaudm@chromium.org

Bug: chromium:1172299
Change-Id: Ie726bb82d4c6648cc9ebd130115ee7ab3d1d551b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2676636Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72558}