This is a reland of 2f3cda58

GetFirstUnusedStackSlot is restored, and used for finding the padding
slot location, if any.

Original change's description:
> [compiler] Rework calculation to start of return slots
>
> - Changes GetOffsetToReturns to take into account return slot padding
>   and argument padding.
> - Changes GetStackParameterDelta to use GetOffsetToReturns for the SP
>   delta calculation.
> - Removes GetFirstUnusedStackSlot.
>
> Change-Id: I13df72e86750c62798bae262f0560cf1d7f981db
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2593306
> Commit-Queue: Bill Budge <bbudge@chromium.org>
> Reviewed-by: Andreas Haas <ahaas@chromium.org>
> Reviewed-by: Georg Neis <neis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#72078}

Change-Id: I954d7e7487728dbd6a545a6c84efb8f8f65d894d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2626979Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72115}