[XLA] HloEvaluator - use TF_ASSIGN_OR_RETURN for HandleCall

Currently HandleCall in the HloEvaluator uses `ConsumeValueOrDie` to get the
value instead of `TF_ASSIGN_OR_RETURN`. This means that the compiler will fail
when running the HLO ConstantFolding pass on a graph with a call instruction
which has an unhandled instruction in its subcompuation.