[QoL] Change ValueRange to ResultRange in gate_op's result getters

[paul0403]

We have some convenience result/operand getters for gate operations, and currently they all return ValueRange.

Nothing is inherently wrong with this, but ResultRange has the replace methods built-in, and ValueRange does not.

To avoid manually casting or looping, we can change the result getters to return ResultRange directly, or provide an overloaded result getter.