This CL factors the parts of the JSGraph that only depend on the
machine part of JSGraph into a separate base class, MachineGraph.
This helps separate the two layers and also allows the MachineGraph
to be constructed without an Isolate, which is needed for fully
asynchronous compilation, a goal for WASM.

R=mstarzinger@chromium.org
CC=jarin@chromium.org, mvstanton@chromium.org

BUG=v8:7721

Change-Id: Ie8bc3de40159332645dcb3cadcee581e1bf9830a
Reviewed-on: https://chromium-review.googlesource.com/1043746Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52990}