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>

src/compiler/nir/glsl_to_nir.cpp

@@ -1852,7 +1852,7 @@ nir_visitor::visit(ir_texture *ir)
       unreachable("not reached");
    }
 
-   instr->sampler = evaluate_deref(&instr->instr, ir->sampler);
+   instr->texture = evaluate_deref(&instr->instr, ir->sampler);
 
    unsigned src_number = 0;