Change a very hot piece of code in TableGen's register unit computations to use bit...

Change a very hot piece of code in TableGen's register unit computations to use bit vectors rather than arrays.

For target descriptions with very large and very dense register files, TableGen
can take an extremely long time to run.  This change makes a dent in that (~15%
in my measurements) by accelerating the single hottest operation with better data
structures.

I believe there's still a lot of room to make this even faster with more global
changes that require replacing some of the existing datastructures in this area
with bit vectors, but that's a more involved change and I wanted to get this
simpler improvement in first.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227562 91177308-0d34-0410-b5e6-96231b3b80d8