[llvm] [StructurizeCFG] Remove one SSAUpdater::AddAvailableValue. NFCI. (PR #115472)

[Jay Foad via llvm-commits]

jayfoad wrote:

Found by experimentation. Does anyone know this code well enough to say for sure whether it is redundant? Or are we missing test coverage?

https://github.com/llvm/llvm-project/pull/115472