glsl: Do not kill dead assignments to buffer variables or SSBO declarations.

If we kill dead assignments we lose the buffer writes.

Also, we never kill UBO declarations even if they are never referenced
by the shader, they are always considered active. Although the spec
does not seem say this specifically for SSBOs, it is probably implied
since SSBOs are pretty much the same as UBOs, only that you can write
to them.

v2:
- Fix the comment (Jordan)

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/glsl/opt_dead_code.cpp

@@ -77,11 +77,13 @@ do_dead_code(exec_list *instructions, bool uniform_locations_assigned)
 
       if (entry->assign) {
 	 /* Remove a single dead assignment to the variable we found.
-	  * Don't do so if it's a shader or function output, though.
+	  * Don't do so if it's a shader or function output or a shader
+	  * storage variable though.
 	  */
 	 if (entry->var->data.mode != ir_var_function_out &&
 	     entry->var->data.mode != ir_var_function_inout &&
-             entry->var->data.mode != ir_var_shader_out) {
+             entry->var->data.mode != ir_var_shader_out &&
+             entry->var->data.mode != ir_var_shader_storage) {
 	    entry->assign->remove();
 	    progress = true;
 
@@ -99,7 +101,8 @@ do_dead_code(exec_list *instructions, bool uniform_locations_assigned)
 	  * stage.  Also, once uniform locations have been assigned, the
 	  * declaration cannot be deleted.
 	  */
-         if (entry->var->data.mode == ir_var_uniform) {
+         if (entry->var->data.mode == ir_var_uniform ||
+             entry->var->data.mode == ir_var_shader_storage) {
             if (uniform_locations_assigned || entry->var->constant_value)
                continue;