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

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> Oops, I can reproduce it. [b686e35](https://github.com/llvm/llvm-project/commit/b686e35ecd81ae76fc517d14e133d44ed7ff9f4b) fixed.

```
Top 5 improvements:
  faiss/IndexIVFFlat.cpp.ll 1879640556 1806714892 -3.88%
  faiss/utils.cpp.ll 1392144764 1341384014 -3.65%
  faiss/IndexIVFFastScan.cpp.ll 7591452840 7321469207 -3.56%
  gromacs/statepropagatordata.cpp.ll 3289712205 3176646034 -3.44%
  rust-analyzer-rs/5ac99zaxn7b9r9xv.ll 2542407788 2477356558 -2.56%
Top 5 regressions:
  faiss/IVFlib.cpp.ll 1451629900 1522431580 +4.88%
  gromacs/constr.cpp.ll 2086259177 2159075037 +3.49%
  gromacs/muParserBase.cpp.ll 3535512461 3655648190 +3.40%
  faiss/IndexIVFAdditiveQuantizer.cpp.ll 758961920 784502318 +3.37%
  faiss/IndexAdditiveQuantizer.cpp.ll 1739627273 1791928381 +3.01%
Overall: 0.00096778%
```



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