gep(nonnull, -1) can not wrap

Missing transformation:

```llvm
define ptr @src(ptr nonnull %p) {
%subone = getelementptr i8, ptr %p, i64 -1
ret ptr %subone
}

define ptr @tgt(ptr nonnull %p) {
%subone = getelementptr nusw i8, ptr %p, i64 -1
ret ptr %subone
}
```

Alive2: https://alive2.llvm.org/ce/z/EtFUxx
This currently doesn't happen: https://godbolt.org/z/vK1noa7ar

This information can help eliminate redundant bounds checks.

Start by reproducing the missing GEP transformation from the issue's LLVM IR with the Alive2 example and the linked Godbolt case. Trace the compiler's handling of a nonnull pointer and a -1 GEP; done means the source form can be transformed to the shown nusw form without invalidating the pointer guarantees.

Written by the indexing model from the issue text.