Fix bug of prune when the program desc has been modified and var.op info is not up-to-date

[kexinzhao]

var.op is usually set to the operator that generates this variable in the prune method in framework.py.

If we firstly prune the program desc to save the original model, and then use float16_transpiler to modify the program desc again. var.op in some cases will be out of date.

So we need to force the prune method to find the latest info about which operator generates the variable in inquiry so that we can find the correct operator as the prune target.