[llvm] [SLP] NFC. Use InstructionsState::valid if users just want to know whether VL has same opcode. (PR #120217)

[Paul Osmialowski via llvm-commits]

pawosm-arm wrote:

I'm seeing this when doing check-llvm:
```
clang++: llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp:821: llvm::Instruction* {anonymous}::InstructionsState::getMainOp() const: Assertion `valid() && "InstructionsState is invalid."' failed.
```
This happens when compiling `llvm/unittests/ADT/DenseMapTest.cpp`, did anyone else observe that too?


https://github.com/llvm/llvm-project/pull/120217