[llvm] [InstCombine] Optimistically allow multiple shufflevector uses in foldOpPhi (PR #114278)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> If I'm reading this correctly, then nothing here actually guarantees that all shufflevectors are going to simplify? You just check that all uses are shufflevectors, but then simplify each individually.

See L1851-1855:
```
     // Be conservative in cases with multiple uses and require all inputs to
     // simplify.

     if (MultipleShuffleVectorUses)
       return nullptr;
```

https://github.com/llvm/llvm-project/pull/114278