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.

Review URL: https://codereview.chromium.org/1513543003

Cr-Commit-Position: refs/heads/master@{#32738}