In the Crankshaft days we (mis)used the Representation to also express
the various internal representations that the compiler understands. But
with TurboFan we now have proper MachineRepresentation and MachineType,
which do that independently. So there's no need to have this in the
Representation class anymore, and instead the Representation class only
needs to deal with the field representations.

Bug: v8:8749, v8:8834, v8:8865
Change-Id: I34ea9558b5fdf20d6c7939b52762eaffd4316b06
Reviewed-on: https://chromium-review.googlesource.com/c/1479954
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59750}