missing offset folding in WASM load/store

WebAssembly backend fails to fold constant GEP offsets into load/store offset parameters when the base pointer comes from a GEP instruction with a variable index in a loop context.

The minimal reproducer [here](https://github.com/lvella/llvm-unopt) is expected to generate, among other instances:
```wasm
...
local.get 1
i64.load offset=8
i64.store offset=2576
```
but instead it generates
```wasm
...
i32.const 2576
i32.add
local.get 1
i32.const 8
i32.add
i64.load
i64.store
```

Start with the minimal reproducer at https://github.com/lvella/llvm-unopt and inspect the WebAssembly backend path that lowers load/store operations when the base pointer comes from a variable-index GEP in a loop. Compare the generated WebAssembly with the expected offset parameters; done means the constant offsets are folded into the load/store instructions rather than emitted as separate additions.

Written by the indexing model from the issue text.