This adds another fixed spill slot to the {WasmCompiledFrame} layout,
holding a reference to the current {WasmInstanceObject}. This slot
allows the stack walker to retrieve instances for WebAssembly frames
without having each code object be coupled to an instance. Hence it
enables sharing code across instances in the future.

R=titzer@chromium.org
BUG=v8:7424

Change-Id: I7fa095c6255754caf564edce4ee7e84dea666783
Reviewed-on: https://chromium-review.googlesource.com/1005516
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52626}