[NFC][RISCV] Keep AVLReg define instr inside VSETVLInfo (#89180)
Currently, the vsetvli pass tracks the define instruction through
`MRI->getVRegDef` due to the SSA form.
This patch keeps the AVLReg DefMI within VSETVLInfo during construction.
And replace `MRI->getVRegDef(AVLReg)` with `getAVLRegDefMI()`.
This information is useful when vsetvli pass live in post-ra situation.
The testcases don't change because the VReg always has a unique def in
SSA.