This patch adds new InstrItinClass for the Hexagon backend.
These timing classes have no effect on V4 instruction set so there is no easy way to test them at this point. However, I'll be adding tests to verify timing classes when add V5 support.
Okay, please remember to add tests when you add v5 support. It need not be an elaborate set though that would be even better :)). We should have tests to ensure that the timing classes are actually consumed and used in codegen.