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}