nir/lower_mediump: Lower FS outputs to 16-bit when the value was upconverted.

nir/lower_mediump: Lower FS outputs to 16-bit when the value was upconverted.

Take this real-world (trimmed) shader:

precision highp float;
in lowp vec4 var_varVertexColor;
layout(location = 0) out vec4 out_FragColor0;
void main() {
    vec4 textureColor0 = vec4(1.000000e+00, 0.000000e+00, 0.000000e+00, 1.000000e+00);
    vec3 color = vec3(1.000000e+00, 1.000000e+00, 1.000000e+00);
    vec4 outColor = vec4(vec3((color).rgb), 1.000000e+00);
    (outColor *= vec4(var_varVertexColor));
    (out_FragColor0 = outColor);
}

After opts, it's just a store from input to output.  If we decide to lower
the input to 16-bit, then as long as the driver can handle 16-bit outputs,
it would be a good idea to demote the output and save the conversions.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18003>