The wasm instance will initially be in kWasmInstanceRegister, and for
each call we also need to put it in that register. Hence, when getting a
new register to cache the instance, prefer that register, if it is
available.

R=thibaudm@chromium.org

Bug: v8:11336
Change-Id: Ie7026c4c7c5e4b825b9ab310839f0273bd3ce7f1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2743885
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73303}