[llvm] [SSAUpdater] Avoid un-necessary SmallVector stores (PR #97820)

[Orlando Cazalet-Hyams via llvm-commits]

https://github.com/OCHyams approved this pull request.

LGTM

Digging around to understand the setup - the IDFCalculator (non-base version) in IteratedDominanceFrontier.h still uses a vector based solution through the namespace `IDFCalculatorDetail` (and it needs to because it modifies the vector). This `IDFCalculatorBase` change makes sense to me - AFAICT only affects InstrRefBasedLDV? Either way, given the returned collection is only iterated over in the this change makes sense.

https://github.com/llvm/llvm-project/pull/97820