[TableGen] Replace two SmallDenseSets with SmallSets.

The key here is HwMode indices. They're going to be small numbers,
contiguous, and only a few different values. I don't think we need
to go through the SmallDenseSet hashing.

A BitVector would be even better, but we don't have the upper
bound here.

GitOrigin-RevId: a623051dc3011fe6f41bddab4024fc213f60282b
1 file changed