glsl: Make read_from_write_only_variable_visitor ignore .length().

.length() on an unsized SSBO variable doesn't actually read any data
from the SSBO, and is allowed on variables marked 'writeonly'.

Fixes compute shader compilation in Shadow of Mordor.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/glsl/ast_to_hir.cpp

@@ -106,6 +106,15 @@ public:
       return found;
    }
 
+   virtual ir_visitor_status visit_enter(ir_expression *ir)
+   {
+      /* .length() doesn't actually read anything */
+      if (ir->operation == ir_unop_ssbo_unsized_array_length)
+         return visit_continue_with_parent;
+
+      return visit_continue;
+   }
+
 private:
    ir_variable *found;
 };