[FIX] Avoid -Wassume error in structural visit VisitImpl (#632)

## Problem

The macOS clang CI job on `main` fails to compile with `-Werror`:

```
include/tvm/ffi/extra/structural_visit.h:575:29: error: the argument to
'__builtin_assume' has side effects that will be discarded [-Werror,-Wassume]
    TVM_FFI_UNSAFE_ASSUME(result.type_index() == TypeIndex::kTVMFFIInt);
```

## Root cause

clang's `__builtin_assume` discards its argument's side effects, so
`-Wassume` rejects any argument that **contains a call expression** —
this is a purely syntactic check, independent of whether the call is
actually pure.

This is a regression from #629, which inlined `result.type_index()`
directly into `TVM_FFI_UNSAFE_ASSUME` to silence `-Wunused-variable`.
The two warnings pull in opposite directions:

| | `-Wunused-variable` | `-Wassume` |
|---|---|---|
| pre-#629 (local var) | ❌ (when assume macro is a no-op) | ✅ |
| #629 (inlined call) | ✅ | ❌ ← CI fails |
| this PR | ✅ | ✅ |

Note `result` here is a C++ `Expected`, so `result.type_index()` is a
**method call**. The other 10 `TVM_FFI_UNSAFE_ASSUME` sites in the tree
operate on the raw `TVMFFIAny` C struct via the plain field
`src->type_index`, which contains no call expression and is therefore
unaffected. This was the only call-expression assume site in the
codebase.

## Fix

Hoist the call back into a local and mark it `[[maybe_unused]]`:

```cpp
[[maybe_unused]] int32_t type_index = result.type_index();
TVM_FFI_UNSAFE_ASSUME(type_index == TypeIndex::kTVMFFIInt);
```

- The assume argument is now a plain variable read → satisfies
`-Wassume`.
- `[[maybe_unused]]` keeps `-Wunused-variable` quiet on configs where
the assume macro compiles away to a no-op → satisfies what #629 was
originally fixing.

## Validation

- `clang++ -std=c++17 -Werror -Wassume -fsyntax-only
src/ffi/extra/structural_visit.cc` (Homebrew clang 22): **passes** after
the change, **fails** before it (reproduces the CI error).
- Confirmed `[[maybe_unused]]` suppresses `-Wunused-variable` when the
assume macro expands to a no-op.
- `clang-format` clean; lines within the 100-col limit.

Author: cyx-6

include/tvm/ffi/extra/structural_visit.h

@@ -572,7 +572,12 @@ class StructuralWalkCallbackVisitorObj : public StructuralVisitorObj {
     if constexpr (order == WalkOrder::kPreOrder) {
       auto result = dispatch_(value, this->def_region_kind());
       TVM_FFI_S_VISIT_MAYBE_EARLY_RETURN_WITH_ERROR_CONTEXT(result, value);
-      TVM_FFI_UNSAFE_ASSUME(result.type_index() == TypeIndex::kTVMFFIInt);
+      // Hoist the call out of TVM_FFI_UNSAFE_ASSUME: clang's -Wassume rejects
+      // arguments that contain a call expression (its potential side effects
+      // would be discarded), while [[maybe_unused]] keeps -Wunused-variable
+      // quiet on configs where the assume macro compiles away.
+      [[maybe_unused]] int32_t type_index = result.type_index();
+      TVM_FFI_UNSAFE_ASSUME(type_index == TypeIndex::kTVMFFIInt);
       if (TVM_FFI_PREDICT_FALSE(details::ExpectedUnsafe::ValueAs<int32_t>(result) ==
                                 WalkResult::kSkipTag)) {
         return details::ExpectedUnsafe::MoveToTVMFFIAny(