[XLA] Refactor parent-fusion-instruction pointer into HloComputation, not HloInstruction.

Presently, each instruction inside a fusion computation contains a pointer to the fusion instruction that contains the computation, which is redundant since this is common across the entire computation.  This leads to lots of places where this pointer must be set when adding an instruction to the fusion computation (and bugs such as b/65177535 when one is missed), as well as code to check that it's set correctly.  In addition, this is simply unnecessary data bloat.

Moreover, the computation itself does not contain a pointer to the fusion instruction that references it, which leads to odd circumlocutions in the HloComputation code that retrieve the fusion instruction from the computation's root instruction.

Thus, this CL moves this pointer into the HloComputation class (replacing the is_fusion_computation_ bool value), and refactor the uses as necessary.

PiperOrigin-RevId: 167039280