[DAGCombiner] Remove unnecessary commonAlignment from CombineExtLoad. (#81705)

The getAlign function for a load returns the commonAlignment of the
"base align" and the offset stored in the MachinePointerInfo.

We're splitting a load here, so we should take the base alignment from
the original load without any offset that may already exist in the
original load. The new load can then maintain its own alignment using
just the base alignment and its own offset.

Noticed by inspection.