glsl: Slight change to the code generated by if-flattening

glsl: Slight change to the code generated by if-flattening

Now the condition (for the then-clause) and the inverse condition (for
the else-clause) get written to separate temporary variables.  In the
presence of complex conditions, this shouldn't result in more code
being generated.  If the original if-statement was

    if (a && b && c && d && e) {
        ...
    } else {
        ...
    }

The lowered code will be

   if_to_cond_assign_then = a && b && c && d && e;
   ...
   if_to_cond_assign_else = !if_to_cond_assign_then;
   ...

Reviewed-by: Eric Anholt <eric@anholt.net>