[ScalarEvolution] Clarify implementation of getPointerBase().

getPointerBase should only be looking through Add and AddRec
expressions; other expressions either aren't pointers, or can't be
looked through.

Technically, this is a functional change. For a multiply or min/max
expression, if they have exactly one pointer operand, and that operand
is the first operand, the behavior here changes. Similarly, if an AddRec
has a pointer-type step, the behavior changes. But that shouldn't be
happening in practice, and we plan to make such expressions illegal.