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

[via llvm-commits]

DianQK wrote:

It's surprising that https://github.com/llvm/llvm-project/pull/98802/commits/b686e35ecd81ae76fc517d14e133d44ed7ff9f4b is making things worse: https://llvm-compile-time-tracker.com/compare.php?from=057abc842e581d01ac0bff435d779e3f506440ad&to=b686e35ecd81ae76fc517d14e133d44ed7ff9f4b&stat=instructions:u.

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