nir/lower_fragcolor: Avoid redundant load_output

At best, this is an extra instruction for NIR to optimize out. At worst,
depending on pass ordering nir_load_output could sneak into the final
NIR, even on drivers that don't support fbfetch.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11255>

src/compiler/nir/nir_lower_fragcolor.c

@@ -65,7 +65,8 @@ lower_fragcolor_instr(nir_builder *b, nir_instr *intr, void *data)
       return false;
    b->cursor = nir_after_instr(&instr->instr);
 
-   nir_ssa_def *frag_color = nir_load_var(b, out);
+   assert(instr->src[1].is_ssa);
+   nir_ssa_def *frag_color = instr->src[1].ssa;
    ralloc_free(out->name);
 
    const char *name = out->data.index == 0 ? "gl_FragData[0]" :