nv50/ir/nir: implement nir_instr_type_tex

nv50/ir/nir: implement nir_instr_type_tex

a lot of those fields are not valid for a lot of tex ops. Not quite sure if
it's worth the effort to check for those or just keep it like that. It seems
to kind of work.

v2: reworked offset handling
    add tex support with indirect R/S arguments
    handle GLSL_SAMPLER_DIM_EXTERNAL
    drop reference in convert(glsl_sampler_dim&, bool, bool)
    fix tg4 component selection
v5: fill up coords args with scratch values if coords provided is less than TexTarget.getArgCount()
v7: prepare for bindless_texture support
v8: don't require C++11 features
v9: convert to C++ style comments
    fix txf with a uniform constant 0 lod

Signed-off-by: Karol Herbst <kherbst@redhat.com>