glsl/lower_output_reads: fix geometry shader output handling with conditional emit

glsl/lower_output_reads: fix geometry shader output handling with conditional emit

Consider a geometry shader that contains code like this:

   some_out = expr;

   if (cond) {
      ...
      EmitVertex();
   } else {
      ...
      EmitVertex();
   }

Both branches should see the correct value of some_out.

Since this is a rather subtle and rare case, I'm submitting a piglit test
for this as well.

GLSL says that the values of output variables are undefined after
EmitVertex(). With this change, the values will now be defined and
unmodified. This may reduce optimization opportunities in the probably
quite rare case where subsequent compiler passes cannot prove that the
value of the output variable is overwritten.

Cc: 13.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>