[llvm] Refactor recomputeLiveIns to operate on whole CFG (PR #79498)

[Nikita Popov via llvm-commits]

nikic wrote:

Reverted in https://github.com/llvm/llvm-project/commit/07a1925b8b92f29880be4030630ed5053783708f, because this introduces a major compile-time regression, see https://llvm-compile-time-tracker.com/compare.php?from=2a068507016e476893b6b1b9c55b255dbae5829e&to=59bf60519fc30d9d36c86abd83093b068f6b1e4b&stat=instructions:u.

https://github.com/llvm/llvm-project/pull/79498