nir/lower_samplers: Use the right memory context for realloc'ing tex sources

As of da5ec2a, we allocate instruction sources out of the instruction
itself.  When we realloc the texture sources we need to use the right
memory context or ralloc will get angry and assert-fail

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

src/glsl/nir/nir_lower_samplers.cpp

@@ -90,7 +90,7 @@ lower_sampler(nir_tex_instr *instr, struct gl_shader_program *shader_program,
                ralloc_asprintf_append(&name, "[%u]", deref_array->base_offset);
             break;
          case nir_deref_array_type_indirect: {
-            instr->src = reralloc(mem_ctx, instr->src, nir_tex_src,
+            instr->src = reralloc(instr, instr->src, nir_tex_src,
                                   instr->num_srcs + 1);
             memset(&instr->src[instr->num_srcs], 0, sizeof *instr->src);
             instr->src[instr->num_srcs].src_type = nir_tex_src_sampler_offset;