Use SmallPtrSet after deprecation (#3329)
Update for llvm-project commit `9b493dcad259` ("[ADT] Deprecate the
redirection from SmallSet to SmallPtrSet (Take 2) (#155078)",
2025-08-23).
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/fc791d14daf0f4b
