[llvm] [SLP] Match poison as instruction with the same opcode (PR #115946)

[via llvm-commits]

zmodem wrote:

We're hitting an assert after this:

```
llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:10881: InstructionCost llvm::slpvectorizer::BoUpSLP::ShuffleCostEstimator::finalize(ArrayRef<int>, ArrayRef<std::pair<const TreeEntry *, unsigned int>>, ArrayRef<int>, unsigned int, function_ref<void (Value *&, SmallVectorImpl<int> &)>): Assertion `SubVectorsMask.size() == CommonMask.size() && "Expected same size of masks for subvectors and common mask."' failed.
```

See https://crbug.com/380805017#comment4 for a reproducer. Can you please take a look?

https://github.com/llvm/llvm-project/pull/115946