[BasicAA] Stop crashing when dealing with pointers > 64 bits.

An alternative (and probably better) fix would be that of
making `Scale` an APInt, and there's a patch floating around
to do this. As we're still discussing it, at least stop crashing
in the meanwhile (added bonus, we now have a regression test for
this situation).

Fixes PR35843.

Thanks to Eli for suggesting the fix and Simon for reporting and
reducing the bug.

llvm-svn: 322467