This particular map is hardly ever queried and has a phased usage pattern (insert,
iterate, query, insert, iterate) so it's a good candidate for a sorted vector and
This significantly reduces the run time of runTargetDesc() in some circumstances.
One llvm-tblgen invocation in my build improves the time spent in runTargetDesc()
from 9.86s down to 0.80s (~92%) without changing the output. The same invocation
also has 2GB less allocation churn.