[RISCV] Add correct Uses, Defs, isReturn to Zcmp (#81039)

* they all do stack adjustments, so they all use and def x2.
* popret and popretz also return
* popretz also defines x10

This adds that to the TD file and updates the PushPopOptimizer to
preserve the extra implicit operands added during frame lowering when
converting to popret(z).