Port bb2a830d
Port 56673804

Original commit messages:
    MachineType is now a class with two enum fields:
    - MachineRepresentation
    - MachineSemantic

    Both enums are usable on their own, and this change switches some places
    from using MachineType to use just MachineRepresentation. Most notably:
    - register allocator now uses just the representation.
    - Phi and Select nodes only refer to representations.

   Store nodes use only MachineRepresentation, not MachineType.

R=jarin@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1523373003

Cr-Commit-Position: refs/heads/master@{#32916}