[llvm] [RISCV] Split out VSETVLIInfo AVL states to be more explicit (PR #89964)

[Piyou Chen via llvm-commits]

BeMg wrote:

Inside `bool RISCVInsertVSETVLI::needVSETVLIPHI`

Maybe the following statement could be dropped due to AVLReg must be Virtual now.
```
  Register AVLReg = Require.getAVLReg();
  if (!AVLReg.isVirtual())
    return true;
```

https://github.com/llvm/llvm-project/pull/89964