[SROA] Teach SROA how to handle pointers from address spaces other than
the default.

Based on the patch by Matt Arsenault, D1764!

I switched one place to use the more direct pointer type to compute the
desired address space, and I reworked the memcpy rewriting section to
reflect significant refactorings that this patch helped inspire.

Thanks to several of the folks who helped review and improve the patch
as well.