[SROA] Simplify the computing of alignment: we only ever need the
alignment of the slice being rewritten, not any arbitrary offset.

Every caller is really just trying to compute the alignment for the
whole slice, never for some arbitrary alignment. They are also just
passing a type when they have one to see if we can skip an explicit
alignment in the IR by using the type's alignment. This makes for a much
simpler interface.

Another refactoring inspired by the addrspace patch for SROA, although
only loosely related.