nir: Remove the const_offset from nir_tex_instr

nir: Remove the const_offset from nir_tex_instr

When NIR was originally drafted, there was no easy way to determine if
something was constant or not.  The result was that we had lots of
special-casing for constant values such as this.  Now that load_const
instructions are SSA-only, it's really easy to find constants and this
isn't really needed anymore.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Rob Clark <robclark@gmail.com>