This renames the macro parameter {index} to {sig_index} to make clear
that this should be the index of a signature.
It also fixes some tests that wrongly passed a function index here (with
no observable difference in the test outcome unfortunately).
It also renames some {f[0-3]} variables to {sig[0-3]} to make it clear
that they refer to signatures.

R=mstarzinger@chromium.org

Bug: v8:10021
Change-Id: Ia4f697acd5d6a2e6f2fb45b680dac20f24ea1b08
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1936471
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65178}