[DSE] Eliminate noop store even through has clobbering between LoadI and StoreI

[DSE] Eliminate noop store even through has clobbering between LoadI and StoreI

For noop store of the form of LoadI and StoreI,
An invariant should be kept is that the memory state of the related
MemoryLoc before LoadI is the same as before StoreI.
For this example:
```
define void @pr49927(i32* %q, i32* %p) {
  %v = load i32, i32* %p, align 4
  store i32 %v, i32* %q, align 4
  store i32 %v, i32* %p, align 4
  ret void
}
```
Here the definition of the store's destination is different with the
definition of the load's destination, which it seems that the
invariant mentioned above is broken. But the definition of the
store's destination would write a value that is LoadI, actually, the
invariant is still kept. So we can safely ignore it.

Differential Revision: https://reviews.llvm.org/D132657