[XLA:GPU] Fix an OOB access bug in `HloFusionAdaptor::GetParameters()`.
[XLA:GPU] Fix an OOB access bug in `HloFusionAdaptor::GetParameters()`.

When considering a `SingleInstructionFusion` that is also a parameter
instruction, the parameter number enclosed within that instruction refers
to the parent computation.

As a result, we shouldn't try to access its operands, but simply enqueue the
instruction itself as a parameter to skip.
