[RISCV] Fix doPRE not checking for ignored AVLs

This fixes a crash introduced in 011a6535
that showed up when compiling with -mrvv-vector-bits=zvl. Beforehand, if a
basic block only contained vmv.s.x the AVL register in VSETVLIInfo would
have been NoRegister since it ignores VL.

In doPRE if AvailableInfo had a register AVL we checked that it dominated,
but coincidentally this failed for NoRegister. Now that the ignored AVL
case is separated out, check for it and bail.

As a side note, it turns out 011a6535 is
less NFC than it seems as we can now do PRE on blocks where AvailableInfo's
AVL is VLMAX.