Changes:
- Add --wasm-loop-unrolling flag. Everything in this CL happens behind
  this flag.
- In decoding, DoReturn does not take returned values as an argument.
  It is now the responsibility of graph-builder-interface.cc to extract
  these values. Note that this is what was already happening in Liftoff.
- In pipeline.cc, add phase to remove loop exits after generating the
  turbofan graph.
- Explicitly disallow calling FallThruTo() on loops.
- Add loop assignments and loop header node to Control type in
  graph-builder-interface.cc. Assign them in Loop().
- Main change: Add loop exit nodes to wasm-generated graphs. For
  details, consult this design doc: https://docs.google.com/document/d/1AsUCqslMUB6fLdnGq0ZoPk2kn50jIJAWAL77lKXXP5g
- Inline PrepareForLoop().

Bug: v8:11298
Change-Id: I65058f1b5df3f862f4a62f4dcb0bd7e1f1dcf4ee
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2621082
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72094}