The {operator==} on {VarState} did not check the spill offset, so when
merging stack states, we forgot to move stack values if both source and
destination were stack slots, but at different offsets.
This CL fixes this by removing the {operator==}, because the semantics
(and use) are not clear, and it's only used in one place anyway.
The equality check was mostly redundant, so inlining it also makes the
code smaller and faster.

R=ahaas@chromium.org

Bug: v8:10702
Change-Id: I6c8b2cfd1002274175c9a17d305692e4631fd7dc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2304574Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68916}