[llvm] [SSAUpdater] Don't use large SmallSets for IDFcalc (PR #97823)

[Nikita Popov via llvm-commits]

nikic wrote:

Does the benefit here really come from switching from SmallPtrSet to SmallDenseSet, or from the added reserve calls? I'd generally expect SmallPtrSet to still be beneficial, even if used in large mode.

https://github.com/llvm/llvm-project/pull/97823