Whenever we spill, num_used_spill_bytes_ is already updated using
RecordSpillSpillSlot, so we don't need to add the number of locals.

Bug: v8:9909
Change-Id: Ieecf957e71e0711be744a3f378d8ae11b941fc5b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1947349Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65306}