Anyref parameters can exist across GC runs. Therefore the GC has to
know where anyref parameters are on the stack so that it can mark them
in its marking phase, and update them in the compaction phase.

Already in a previous CL we grouped all anyref parameters so that they
can be found more easily in a stack frame, see
https://crrev.com/c/1371827. In this CL we implement the stack scanning
itself.

Note that anyref parameters are not scanned while iterating over the
caller's frame (to which they actually belong), but while iterating
over the callee's frame. The reason is that with tail-calls, only the
callee knows how many tagged stack parameters (aka anyref parameters)
there are.

R=mstarzinger@chromium.org
also-by=mstarzinger@chromium.org

Bug: v8:7581
Change-Id: I7a41ce11d06c0d420146fdb0bb8d5606f28824d7
Reviewed-on: https://chromium-review.googlesource.com/c/1424955
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59099}