[DFAPacketizer] Allow up to 64 functional units

Summary:
To drive the automaton we used a uint64_t as an action type. This
contained the transition's resource requirements as a conjunction:

  (a OR b) AND (b OR c)

We encoded this conjunction as a sequence of four 16-bit bitmasks.
This limited the number of addressable functional units to 16, which
is quite low and has bitten many people in the past.

Instead, the DFAEmitter now generates a lookup table from InstrItinerary
class (index of the ItinData inside the ProcItineraries) to an internal
action index which is essentially a dense embedding of the conjunctive
form. Because we never materialize the conjunctive form, we no longer
have the 16 FU restriction.

In this patch we limit to 64 functional units due to using a uint64_t
bitmask in the DFAEmitter. Now that we've decoupled these representations
we can increase this in future.

Reviewers: ThomasRaoux, kparzysz, majnemer

Reviewed By: ThomasRaoux

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69110