Avoid the indirection via the signature, instead specialize per
signature. On a microbenchmark run locally, this speeds up Liftoff by
~5% and validation by ~15%.

R=titzer@chromium.org

Bug: v8:8423
Change-Id: Ia0e9ab0dcaa759e4ed4dcd46d2e6f16e65790915
Reviewed-on: https://chromium-review.googlesource.com/c/1373778Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58241}