[wasm] Enable loop unrolling

We experimentally globally enable loop unrolling for wasm code. This
might be reverted based on the results of perf bots.

Additional change: Add LoopExitValue to Int64Lowering, plus a small
simplification.

Bug: v8:11298

Change-Id: Iaf2829e80f948d70c5fb6ed7c974db7f59265fa3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2718611


Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73072}