Separate out MVT in a separate header file: MachineValueType.h

The idea behind this split of ValueTypes.h, is to make it easier to
ensure that stuff after type legalization only use MVT (rather than
EVT), by watching include dependencies.

Reviewed By: Tim Northover

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203926 91177308-0d34-0410-b5e6-96231b3b80d8