Some architectures used {kConstantStackSpace}, others used
{kInstanceOffset}. This CL unifies it to {kInstanceOffset} and uses that
constant consistently (in {GetInstanceOperand}).

R=zhin@chromium.org

Bug: v8:10019
Change-Id: Ia2b6908e289591e2dbc48e559e11407877b7c4ad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2000146Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65780}