This is a reland of 6b4dc039
Original change's description:
> [wasm] Refactor function body decoder
> 
> This refactoring separates graph building from wasm decoding. The
> WasmGraphBuilder is just a consumer of the decoded information.
> Decoding without any consumer (i.e. just validation) gets 16% faster by
> this refactoring, because no TFNode* have to be stored in the value
> stack, and all dynamic tests to determine whether the graph should be
> build are gone (measured on AngryBots; before: 110.2 +- 3.3ms, after:
> 92.2 +- 3.1 ms).
> 
> This new design will allow us to also attach other consumers, e.g. a
> new baseline compiler.
> 
> R=titzer@chromium.org
> 
> Bug: v8:6600
> Change-Id: I4b60f2409d871a16c3c52a37e515bcfb9dbb8f54
> Reviewed-on: https://chromium-review.googlesource.com/571010
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Ben Titzer <titzer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#47671}

TBR=titzer@chromium.org

Bug: v8:6600
Change-Id: Idd867c5a1917437de5b6e3de5917cc1c9f194489
Reviewed-on: https://chromium-review.googlesource.com/640591Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47678}