[llvm] [RISCV] Eliminate dead li after emitting VSETVLIs (PR #65934)

[Luke Lau via llvm-commits]

================
@@ -1262,9 +1262,17 @@ void RISCVInsertVSETVLI::emitVSETVLIs(MachineBasicBlock &MBB) {
       if (RISCVII::hasVLOp(TSFlags)) {
         MachineOperand &VLOp = MI.getOperand(getVLOpNum(MI));
         if (VLOp.isReg()) {
+          Register Reg = VLOp.getReg();
+          MachineInstr *VLOpDef = MRI->getVRegDef(Reg);
+
           // Erase the AVL operand from the instruction.
           VLOp.setReg(RISCV::NoRegister);
           VLOp.setIsKill(false);
+
+          // If the old VLOp was only used by MI, it's dead.
----------------
lukel97 wrote:

Maybe just a clarification on why we're specifically checking for ADDI

```suggestion
          // If the AVL was an immediate > 31, then it would have been emitted as an ADDI. However, the ADDI might not have been used in the vsetvli, or a vsetvli might not have been emitted, so it may be dead now.
```

https://github.com/llvm/llvm-project/pull/65934