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

[Matt Arsenault via llvm-commits]

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

I'm guessing this would only be needed if there were an unconditional branch from the entry block. But in that case, it would never be necessary to insert a phi there so this seems fine 

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