[llvm] [JumpThreading] Use SmallPtrSet (NFC) (PR #95674)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> Actually, it would be even better to switch this to SmallPtrSet.

Thanks!  `SmallPtrSet` saves a little more allocations.

https://github.com/llvm/llvm-project/pull/95674