MachineType now tracks both the representation and the value type of machine quantities and is used uniformly throughout TurboFan.

These types can now express uint8, int8, uint16, and int16, i.e. signed and unsigned smallish integers. Note that currently only uint8 and uint16 are implemented in the TF backends.

R=bmeurer@chromium.org, mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/470593002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23122 ce2b1a6d-e550-0410-aec6-3dcde31c8c00