[cleanup][maglev] Fix pushed_register_base location

I forgot to remove the -1 when hoisting it.

Bug: v8:7700
Change-Id: I407d387058ef476ae2359f8c3815d6a70fff1b97
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3791904
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82037}

src/execution/frames.cc

@@ -1467,7 +1467,7 @@ void MaglevFrame::Iterate(RootVisitor* v) const {
     while (tagged_register_indexes != 0) {
       int index = base::bits::CountTrailingZeros(tagged_register_indexes);
       tagged_register_indexes &= ~(1 << index);
-      FullObjectSlot spill_slot = pushed_register_base - index - 1;
+      FullObjectSlot spill_slot = pushed_register_base - index;
       VisitSpillSlot(isolate(), v, spill_slot);
     }
   }