Add a SmallSet and use it in LocalGraph. NFC (#4188)

A SmallSet starts with fixed storage that it uses in the simplest
possible way (linear scan, no sorting). If it exceeds a size then it
starts using a normal std::set. So for small amounts of data it
avoids allocation and any other overhead.

This adds a unit test and also uses it in LocalGraph which provides
a large amount of additional coverage.

I also changed an unrelated data structure from std::map to
std::unordered_map which I noticed while doing profiling in
LocalGraph. (And a tiny bit of additional refactoring there.)

This makes LocalGraph-using passes like ssa-nomerge and
precompute-propagate 10-15% faster on a bunch of real-world
codebases I tested.

0 files changed, 0 insertions, 0 deletions