[SelectionDAG] Always preserve offset in MachinePointerInfo

Previously, getWithOffset() would drop the offset if the base was null.
Because of this, MachineMemOperand would return the wrong result from
getAlign() in these cases.  MachineMemOperand stores the alignment of
the pointer without the offset.

A bunch of MIR tests changed because we print the offset now.

Split off from D77687.

Differential Revision: https://reviews.llvm.org/D78049