glsl: Remove spurious assertions

It's inside an if-statement that already checks that the variables are
not NULL.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>

src/compiler/glsl/opt_copy_propagation.cpp

@@ -355,8 +355,6 @@ ir_copy_propagation_visitor::add_copy(ir_assignment *ir)
       if (lhs_var->data.mode != ir_var_shader_storage &&
           lhs_var->data.mode != ir_var_shader_shared &&
           lhs_var->data.precise == rhs_var->data.precise) {
-         assert(lhs_var);
-         assert(rhs_var);
          _mesa_hash_table_insert(acp, lhs_var, rhs_var);
       }
    }