Port adb7e37b

Original Commit Message:

    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.

R=bmeurer@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: Ie3c8062786d5fd42872e22be01cea45d719ea0a4
Reviewed-on: https://chromium-review.googlesource.com/c/1479972Reviewed-by: Junliang Yan <jyan@ca.ibm.com>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#59767}