tfdbg: fix a bug in graph validation related to tf.while_loops

CL/147488620 fixed a bug where the debugger would hang at Enter and NextIteration nodes under certain conditions. But it introduced another bug where the debug dumps from Enter and NextIteration may get generated later than downstream nodes in the tf.while_loop body, causing "causility violation" during debug_data.DebugDumpDir's validation process under certain conditions (e.g., backpropagation on a dynamic_rnn). This CL fixes that by excluding Enter and NextIteration nodes from the validation process.

Fixes: #8337
Change: 151787432