[llvm] [NFC][RISCV] Keep AVLReg define instr inside VSETVLInfo (PR #89180)

[Piyou Chen via llvm-commits]

BeMg wrote:

Update
1. Use the const reference for `getAVLDefMI`
2. Keep both `AVLReg` and `AVLRegDefMI` inside `VSETVLInfo`
3. Rename some function
4. Move `AVLReg AVLRegDefMI` assert check into `getAVLDefMI`, `setAVLRegDef` and `getAVLReg`

https://github.com/llvm/llvm-project/pull/89180