Move MachineRepresentation to machine-type.h and rename to MachineType in preparation for merging it with RepType.

R=bmeurer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/456333002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23051 ce2b1a6d-e550-0410-aec6-3dcde31c8c00