We calculate the spill offset of a value by examining the top of the
stack, checking its offset, and adding the size of the value.

The offset is passed around for creations of other VarState, but is
otherwise not used in any meaningful way yet.

Bug: v8:9909
Change-Id: Id06f0e1cf932ba63dc291c94a3e513f4d815c554
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1913501
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64974}