glsl: Properly throw compile-time errors for conflicting FS output writes.

glsl: Properly throw compile-time errors for conflicting FS output writes.

We were checking for these at link time previously, which is not as
early as mandated, and would actually fail to detect conflicting
writes if dead code removal removed some writes.

Fixes failures in piglit
glsl-*/compiler/fragment-outputs/write-gl_Frag*

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