Design doc: https://docs.google.com/document/d/1AsUCqslMUB6fLdnGq0ZoPk2kn50jIJAWAL77lKXXP5g/

Currently, wasm loop unrolling is disabled by default. We intend to
further investigate its compilation time cost and running time benefits
before enabling it.

Additional changes:
- Introduce LoopFinder::FindUnnestedLoopFromHeader() as a lightweight
  loop analysis.
- Move EliminateLoopExit into LoopPeeling and expose it.
- Introduce loop_info_ field into WasmGraphBuildingInterface, fill it
  up in Loop().
- Break after encountering the first loop in BuildNestedLoopExits.
- Introduce struct WasmLoopInfo. A WasmLoopInfo vector is instantiated
  in ExecuteTurbofanWasmCompilation, passed to BuildGraphForWasmFunction
  to be filled up by WasmGraphBuildingInterface, and then passed to
  GenerateCodeForWasmFunction to be used in WasmLoopUnrollingPhase.
- Introduce WasmLoopUnrollingPhase and insert it into the wasm
  compilation pipeline.
- Fix an issue where exception values were not wrapped in
  WasmGraphBuilderInterface.
- Update --wasm-loop-unrolling flag description.

Bug: v8:11298
Change-Id: I4b57cf2ea8520931f60769f843ffd57b3ca6399b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2697349
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73009}