This patch changes many callers of GetStackOffsetFromIndex to directly
use the offset that is stored in the VarState (and other structures).

The tricky part here is that in all archs, GetStackSlotOffset no longer
relies on kFirstStackSlotOffset, because the offset stored in VarState
is relative to the constant space (instance offset), and not offset of
the first stack slot.

For example, for slot 0, the offset was also 0, because it was relative
to the first stack slot offset (which in x64 is fp-24). With this
change, the offset of slot 0 is now 8, but since GetStackSlotOffset is
relative to fp-16, it ends up being fp-24 still.

Because of this change, callers of GetStackOffsetFromIndex need to add
1 to whatever index they were passing. Instead of doing that, we change
GetStackOffsetFromIndex to add 1 inside the body.

After this change, the only callers of GetStackOffsetFromIndex will be
inside of FillStackSlotsWithZero, because they still rely on index to
keep track of how many params were processed, and also how many locals
there are in order to zero those slots, and these is relied on by
RecordUsedSpillSlot to allocate sufficient stack space.

Bug: v8:9909
Change-Id: I52aa4572950565a39e9395192706a9934ac296d4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1925524
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65109}