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

[Jeremy Morse via llvm-commits]

jmorse wrote:

Added a `capacity` method as it's extremely simple and added some more test coverage. As a result, I've tweaked `reserve` to always allocate 128 elements whenever it grows from a small size, to match the behaviour of `insert_imp_big`. Also a filter for zero-sized reserves that was going to mess with some of the logic.

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