This improves the stack layout of {WasmCompiledFrame} frames built by
Liftoff so that the first spill slot immediately follows the frame
marker. We will rely on this in the future when we expect the first
spill slot to always hold a {WasmInstanceObject} reference.

R=clemensh@chromium.org

Change-Id: I2babe8a813af23f3b5bc139a2b0b334072625f7b
Reviewed-on: https://chromium-review.googlesource.com/1006615Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52543}