port bb2a830d (r32738)

  original commit message:
  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.

BUG=

Review URL: https://codereview.chromium.org/1520793002

Cr-Commit-Position: refs/heads/master@{#32778}