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

[Nikita Popov via llvm-commits]

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

LGTM. Not seeing any impact on average cases (https://llvm-compile-time-tracker.com/compare.php?from=e790c97f65687bece901072cb4f6935061785536&to=63bfce66db9ade00b3140e637d83ca41e48909bc&stat=instructions:u), but will help degenerate ones.

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