Fix assert syntax for macOS compiler builds.

```
/Users/runner/work/iree/iree/compiler/src/iree/compiler/Codegen/Dialect/VectorExt/IR/VectorExtAttrs.cpp:221:12: error: implicit conversion turns string literal into bool: 'const char[44]' to 'bool' [-Werror,-Wstring-conversion]
    assert("Invalid dimension! Expected lane dimension.");
    ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

compiler/src/iree/compiler/Codegen/Dialect/VectorExt/IR/VectorExtAttrs.cpp

@@ -218,7 +218,7 @@ uint64_t LayoutAttr::getShuffleOffset(int64_t reductionDim) {
              getShape(LayoutDimension::LANEY).value_or(0);
     break;
   default:
-    assert("Invalid dimension! Expected lane dimension.");
+    assert(false && "Invalid dimension! Expected lane dimension");
     break;
   }
   return offset;