This overlays the "arguments count" and the "deoptimization index"
fields within a safepoint entry. It allows for a larger value of
arguments count to be stored in entries that do not contain a
deoptimization index. Currently the arguments count is unused in
TurboFan, but it will be used to handle reference type arguments in
WebAssembly code.

R=ahaas@chromium.org
BUG=v8:7581

Change-Id: I1e1d5af7e69288f046dc327de5d6e0466fc9ceaf
Reviewed-on: https://chromium-review.googlesource.com/c/1371829
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58224}