[XLA:GPU] Move buffer assignment logic to MemcpyFusion::Emit.
[XLA:GPU] Move buffer assignment logic to MemcpyFusion::Emit.

GetFusionEmitter should be responsible only for matching the correct emitter. If buffer assignment fails, there is no way to fix that and we propagate status to the caller anyway, so it's better to do it in MemcpyFusion::Emit.
