[llvm] [SimplifyCFG] Speculatively execute empty BBs with multiple predecessors (PR #120905)

[via llvm-commits]

goldsteinn wrote:

> The current implementation introduces significant compile-time impact :(
> 
> http://llvm-compile-time-tracker.com/compare.php?from=9423961f259f49007dc9cdea7344094a6fdf1afb&to=fe8f09804447631d2a99a626ed582f1398e838ef&stat=instructions:u

Is there a sense for if the compile time impact is from time spent in `speculativelyExecuteEmptyBB` or as a consequence of the IR transformations.

https://github.com/llvm/llvm-project/pull/120905