[XLA] Add ReducePrecisionInsertion pass.

This new HLO pass, intended for experimental purposes rather than optimization, inserts ReducePrecision instructions (with user-specified bitsizes) after all instructions of opcode types specified by the user.  This makes it possible to do experiments on the numerical effects storing intermediate values in reduced precision without changing the HLO graph definition.

PiperOrigin-RevId: 161117760