Avoid creating an immutable map in the Automaton class.

Summary:
In the DFAPacketizer we copy the Transitions array
into a map in order to later access the transitions
based on a "Current State/Action" pair as a key.
This map lives in the Automaton object used by the DFAPacketizer.
It is never changed during the life of the object after
having been created during the creation of the Automaton
itself.

This map creation can make the creation of a DFAPacketizer
quite expensive if the target contains a considerable
amount of transition states.

Considering that TableGen already generates a
sorted list of transitions by State/Action pairs
we could just use that directly in our Automaton
and search entries with std::lower_bound instead of copying
it in a map and paying the execution time and memory cost.

Reviewers: jmolloy, ThomasRaoux

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72682