[DFAPacketizer] Allow namespacing of automata per-itinerary

The Hexagon itineraries are cunningly crafted such that functional units between
itineraries do not clash. Because all itineraries are bundled into the same DFA,
a functional unit index clash would cause an incorrect DFA to be generated.

A workaround for this is to ensure all itineraries declare the universe of all
possible functional units, but this isn't ideal for three reasons:
  1) We only have a limited number of FUs we can encode in the packetizer, and
     using the universe causes us to hit the limit without care.
  2) Silent codegen faults are bad, and careful triage of the FU list shouldn't
     be required.
  3) Smooshing all itineraries into the same automaton allows combinations of
     instruction classes that cannot exist, which bloats the table.

A simple solution is to allow "namespacing" packetizers.

Differential Revision: https://reviews.llvm.org/D66940

llvm-svn: 370508