nir: split SSBO min/max atomic instrinsics into signed/unsigned versions

nir: split SSBO min/max atomic instrinsics into signed/unsigned versions

NIR is typeless so this is the only way to keep track of the
type to select the proper atomic to use.

v2:
  - Use imin,imax,umin,umax for the intrinsic names (Connor Abbott)
  - Change message for unreachable paths (Michael Schellenberger)

Tested-by: Markus Wick <markus@selfnet.de>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>