This CL is the start to implement reference types support in Liftoff.
As a first step this CL implements the ref.null instruction, and allows
reference types as return values. This allows register allocation to be
extended to support reference types, and also adds an easy way to get a
reference type value for tests. Additionally with ref.null we don't have
to worry about garbage collection because 'null' is an immovable object
and therefore does not have to be scanned by the GC.

R=thibaudm@chromium.org

Bug: v8:7581
Change-Id: I5785dcf522c0d9881e1386f2d8b5d8560a16225c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2352784
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69556}