TF-TRT: Run constant folding after layout optimizer, and run the optimizers only once.

Rational: layout optimizer adds Const nodes followed by Reshape nodes, and TF-TRT supports reshape now, so TF should do const folding instead of letting TF-TRT do do that. Also, running TRT optimizer twice is buggy since it'll redo the conversion and try to add TF functions and TRTEngineOps with conflict names to the graph.
PiperOrigin-RevId: 219815498