nir/tex_instr: Rename sampler to texture

nir/tex_instr: Rename sampler to texture

We're about to separate the two concepts.  When we do, the sampler will
become optional.  Doing a rename first makes the separation a bit more
safe because drivers that depend on GLSL or TGSI behaviour will be fine to
just use the texture index all the time.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>