This CL adds support for "secondary parameter locations" for
the WASM instance parameter. Since this parameter is spilled onto
the stack by the function prologue, it is not necessary to allocate
additional spill slots for it. This saves both code space and
runtime.

R=mstarzinger@chromium.org

Change-Id: I440e2c58cd2a227be8a7f386df5193d8fb729fd1
Reviewed-on: https://chromium-review.googlesource.com/1075056
Commit-Queue: Ben Titzer <titzer@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53387}