Allow data prefetch into non-default address space

I am playing with the LoopDataPrefetch pass and found out that it
bails to work with a pointer in a non-zero address space. This
patch adds the target callback to check if an address space is to
be considered for prefetching. Default implementation still only
allows address space 0, so this is NFCI.

This does not currently affect any known targets, but seems to be
generally useful for the future.

Differential Revision: https://reviews.llvm.org/D129795