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

We currently use AVLIsReg to represent VLMAX as well as a dummy value
for
whenever the VL is ignored by vmv.x.s. This splits them out into
separate
states so that AVLIsReg is always a virtual register and should help
with
tracking the definition inside VSETVLIInfo directly in #89180.

This is almost an NFC but it sets the kill flag for x0 in more places.