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

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> IR diff LGTM. I am running compile-time tests :)
> 

```
Top 5 improvements:
  yosys/opt_muxtree.ll 6478612386 5814367885 -10.25%
  proxy/proxy_lifetime_tests.cpp.ll 7675508865 6910976274 -9.96%
  luau/InsertionOrderedMap.test.cpp.ll 1564039427 1431728556 -8.46%
  yosys/extract_reduce.ll 6042277718 5548421964 -8.17%
  yosys/attrmvcp.ll 3867732946 3646575352 -5.72%
Top 5 regressions:
  cmake/cmGeneratorExpressionLexer.cxx.ll 280987017 355193810 +26.41%
  hermes/StackPromotion.cpp.ll 5084729293 5932637578 +16.68%
  verilator/V3GraphPathChecker.cpp.ll 384704949 442379033 +14.99%
  yosys/shregmap.ll 6984227787 7798033142 +11.65%
  assimp/Exporter.cpp.ll 2987022380 3312585333 +10.90%
Overall: 0.01310176%
```

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