This class was defined in function-body-decoder.cc, but it's not an
implementation of function body decoding, but rather the interface
between the decoder and the WasmGraphBuilder. Hence move it out to its
own file.

R=titzer@chromium.org, mstarzinger@chromium.org

Bug: v8:8238
Change-Id: Ib9bf47e90a3683f578b30b6de74d01da81b2be93
Reviewed-on: https://chromium-review.googlesource.com/c/1278391
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56602}