[llvm] [SLP] NFC. Replace MainOp and AltOp in TreeEntry with InstructionsState. (PR #120198)

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

> Hi,
> 
> After recommit of this patch I still see the crash `` clang: ../lib/Transforms/Vectorize/SLPVectorizer.cpp:821: Instruction *(anonymous namespace)::InstructionsState::getMainOp() const: Assertion `valid() && "InstructionsState is invalid."' failed. `` for some downstream tests that https://lab.llvm.org/buildbot/#/builders/56/builds/15893 hit after the first version of this patch. I'll see if I can extract a reproducer but I'm not sure I'll manage before the weekend.

@HanKuanChen :
Reduced opt reproducer:
```opt -passes slp-vectorizer bbi-102823.ll -o /dev/null```
[bbi-102823.ll.gz](https://github.com/user-attachments/files/18378640/bbi-102823.ll.gz)


https://github.com/llvm/llvm-project/pull/120198