[llvm] [InstCombine] Improve `foldDeadPhiWeb` compile time (PR #158057)

[Nikita Popov via llvm-commits]

nikic wrote:

Clang fails to bootstrap with this change: https://llvm-compile-time-tracker.com/show_error.php?commit=bc97a194ac4576b4bbe448f73314f746d3411a0d

https://github.com/llvm/llvm-project/pull/158057