"Isolate" must-be-constant side effecting operations

I first tried to fix this issue in cr/209996730 but didn't quite fix the problem
for for XLA_* devices.  A node assigned to an XLA_* device must be compiled so
the cr/209996730 fix of simply not compiling the nodes doesn't generalize to
XLA_* devices.  Instead we now "isolate" these nodes, only putting them in a
trivial one-node cluster.  For non-XLA devices even this trivial cluster is
ignored because of flags->tf_xla_min_cluster_size.

I was initially considering a more principled data-flow-analysis based solution
but then decided the upfront work isn't worth it until I see a clear motivating
example.

PiperOrigin-RevId: 213531437

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