[SeparateConstOffsetFromGEP] Reorder trivial GEP chains to separate constants (#73056)

In this case, a trivial GEP chain has the form:

```
%ptr = getelementptr sameType, %base, constant
%val = getelementptr sameType, %ptr, %variable
```

That is, a one-index GEP consumes another (of the same basis and result
type) one-index GEP, where the inner GEP uses a constant index and the
outer GEP uses a variable index. For chains of this type, it is trivial
to reorder them (by simply swapping the indexes). The result of doing so
is better AddrMode matching for users of the ultimate ptr produced by
GEP chain.

Future patches can extend this to support non-trivial GEP chains (e.g.
those with different basis types and/or multiple indices).