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

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

Hi @HanKuanChen 

What about the crash
```clang: /b/1/clang-x86_64-debian-fast/llvm.src/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:821: llvm::Instruction *(anonymous namespace)::InstructionsState::getMainOp() const: Assertion `valid() && "InstructionsState is invalid."' failed.```
seen in the failed build bots?

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