nir/lower_blend: Fix SNORM logic ops

We need to sign extend. Incidentally this means the iand above is useless for
SNORM.

Fixes arb_color_buffer_float-render with GL_RGBA8_SNORM.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20016>

src/compiler/nir/nir_lower_blend.c

@@ -313,6 +313,8 @@ nir_blend_logicop(
    if (util_format_is_unorm(format)) {
       out = nir_format_unorm_to_float(b, out, bits);
    } else if (util_format_is_snorm(format)) {
+      /* Sign extend before converting so the i2f in snorm_to_float works */
+      out = nir_format_sign_extend_ivec(b, out, bits);
       out = nir_format_snorm_to_float(b, out, bits);
    } else {
       assert(util_format_is_pure_integer(format));