More or less directly copied from IR/Verifier.cpp.
On second thought, I'm not sure this is necessary. The MIRParser won't ever produce a 0 base alignment MMO. This can probably be replaced with an assert in the MMO constructor
It turns out the MachineMemOperand constructor already asserts on a zero alignment. The vector check is potentially useful, but given this doesn't cover SelectionDAG anyway (where I'm currently focused), I'm deferring it until a future point.