[DL] Optimize address space zero lookup (NFC)

Information for pointer size/alignment/etc is queried a lot, but
the binary search based implementation makes this fairly slow.

Add an explicit check for address space zero and skip the search
in that case -- we need to specially handle the zero address space
anyway, as it serves as the fallback for all address spaces that
were not explicitly defined.

I initially wanted to simply replace the binary search with a
linear search, which would handle both address space zero and the
general case efficiently, but I was not sure whether there are
any degenerate targets that use more than a handful of declared
address spaces (in-tree, even AMDGPU only declares six).