During a frame merge we call ForEachValue that "get"s every
live register, including the virtual accumulator. That currently
fails since we need to check if the register is the virtual
accumulator and set/get the accumulator field in InterpreterFrameState.

The virtual accumulator slot in RegisterFrameArray (the same as the
return address in a live frame) is actually unused.
So we can use this slot for the InterpreterFrameState's accumulator,
instead of a separate field.

Bug: v8:7700
Change-Id: Ife33946a4f9c58ca1f4eadeb587f9880f6fb2afc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3536648
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79553}