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

[via llvm-commits]

DianQK wrote:

> BTW, I think most of dead blocks are created by JumpThreading. Can we avoid this to further improve compilation time?

I haven't read the JumpThreading code yet, if it can't be handled within it, perhaps we could create some sort of simple matching pattern to hand over to SimplifyCFG?

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