[llvm][ADT] Add PointerUnion benchmarks. NFC. (#187874)
Add microbenchmarks for `PointerUnion`'s `isa` and `isNull` operations
across union sizes of 2, 4, and 8 types. This it to establish baseline
performance numbers before making changes to the implementation.
I plan to refactor the implementation a bit and add support for more
variants using sparse encoding.
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
