[RISCV] Assert only valid AVLs in doLocalPostpass are X0 or virtual regs. NFC

In vxrm.mir we were running RISCVInsertVSETVLI on pseudos that already had
vsetvlis inserted and their AVLs set to $noreg. (This happened to work
since doLocalPostpass got rid of the extra vsetvli)

This removes the vsetvlis from the test and enforces that the only valid
AVLs we work with are either X0 or virtual registers (or $noreg before
emitVSETVLIs), since we don't handle physical registers properly in
doLocalPostpass.