[SROA] Use the members for New{Begin,End}Offset in the rewrite helpers
rather than passing them as arguments.

While I generally prefer actual arguments, in this case the readability
loss is substantial. By using members we avoid repeatedly calculating
the offsets, and once we're using members it is useful to ensure that
those names *always* refer to the original-alloca-relative new offset
for a rewritten slice.

No functionality changed. Follow-up refactoring, all toward getting the
address space patch merged.