Calculate the number of bytes of the stack frame used in
PatchPrepareStackFrame using the size of the spill instead of the number
of slots.

We only need the number of bytes spilled (without adding the number of
locals) because whenever we spill, we already track the largest offset,
with RecordUsedSpillSlot. GetTotalFrameSlotCount can also be changed to
remove the num_locals, in a future patch.

Change-Id: I08fe3e81eaebf5f2cf1e11292645663474483447
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1945944
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65303}