Fix a few issues with HloValue and dataflow analysis identified when debugging...

Fix a few issues with HloValue and dataflow analysis identified when debugging the causes for the rollback of the new copy insertion (cl/174423881):

(1) Mark values for deletion during dataflow propagation and delete later, rather than delete immediately. It was possible for a value to be deleted (a phi is optimized away), and still have references to it in the value sets.

(2) Make call/while and root instructions explicit uses of the values which reach them. This subsumes the need for the HloValue::live_out_of_computation_ property which was buggy (which computation is it live-out of?).

(3) Delete unused methods HloValue::RecomputeUses and HloValue::RemovePosition.

PiperOrigin-RevId: 175765613