nir: Add explicit signs to image min/max intrinsics

nir: Add explicit signs to image min/max intrinsics

This better matches all the other atomic intrinsics such as those for
SSBOs and shared variables where the sign is part of the intrinsic
opcode.  Both generators (GLSL and SPIR-V) know the sign from the type
of the image variable or handle.  In SPIR-V, signed min/max are separate
opcodes from unsigned.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>