[llvm] [RISCV][VLOPT] Allow propagation even when VL isn't VLMAX (PR #112228)

[Luke Lau via llvm-commits]

================
@@ -759,29 +787,45 @@ bool RISCVVLOptimizer::tryReduceVL(MachineInstr &OrigMI) {
     MachineInstr &MI = *Worklist.pop_back_val();
     LLVM_DEBUG(dbgs() << "Trying to reduce VL for " << MI << "\n");
 
-    std::optional<Register> CommonVL;
+    std::optional<const MachineOperand *> CommonVL;
----------------
lukel97 wrote:

For pointers you don't need to wrap it with std::optional, I think you can just do
```suggestion
    const MachineOperand * CommonVL = nullptr;
```

And check if that's null instead. So the signature of checkUsers would be 
```c++
bool RISCVVLOptimizer::checkUsers(const MachineOperand *&CommonVL, MachineInstr &MI)
```

https://github.com/llvm/llvm-project/pull/112228