[llvm] [SimplifyCFG] Select the first instruction that we can handle in `passingValueIsAlwaysUndefined` (PR #98802)

[via llvm-commits]

DianQK wrote:

IMO, checking all the uses works too: https://llvm-compile-time-tracker.com/compare.php?from=181e4c6291c94a38c0ee89d2128f8d70b15d2d23&to=1d88f9270188c0ebc8bc89f1cee6a9f5c2045974&stat=instructions:u.

https://github.com/llvm/llvm-project/pull/98802