[XLA:GPU] Store fusion_roots and fusion_heroes as HloInstructionAdaptor in HloFusionAnalysis.
[XLA:GPU] Store fusion_roots and fusion_heroes as HloInstructionAdaptor in HloFusionAnalysis.

HloFusionAnalysis works on fused and unfused HLO, so it's more logical to instruction adaptors that point to the fusion adaptor.
