[RISCV] Use PromoteSetCCOperands to promote operands for UMAX/UMIN during type...

[RISCV] Use PromoteSetCCOperands to promote operands for UMAX/UMIN during type legalization. (#82716)

For RISC-V, we were always choosing to sign extend when promoting
i32->i64. If the promoted inputs happen to be zero extended already, we
should use zero extend instead. This is what we do for SETCC.