Re: Missed compiler optimization issue in function select_rtable_names_for_explain

I mean that the stores with value "0" after the memset are dead:
```     dpns.subplans = NIL;    dpns.ctes = NIL;    dpns.appendrels = NULL;
```
since the memset has written zeroes to the object "dpns", and these members are known to be zero.

They are known to be zero, but that's not entirely equivalent though is it?
NIL is defined as ((List *) NULL) and NULL is typically defined as ((void *)
0), so sizeof(0) would be the size of an int and sizeof(NULL) would be the size
of a void pointer.

The type or size doesn't matter here. At IR or assembly level, they are all zeroes. 

My main point is that "dpns.xxx" are filled with zeroes by the memset firstly, and overwriting them with zeroes in the following stores is redundant. LLVM cannot remove the redundant overwrites due to the initialization order. If we adjust the order of the initialization of "dpns.xxx", the compiler can remove such stores.

Does my explanation make sense to you?

Best regards, Hongyu.