Method overrides do not re-trigger compilation

```julia
julia> kernel() = child()
kernel (generic function with 1 method)

julia> child() = 0
child (generic function with 1 method)

julia> native_code_llvm(kernel, Tuple{})
; @ REPL[1]:1 within `kernel`
; Function Attrs: alwaysinline
define i64 @julia_kernel_755() local_unnamed_addr #0 {
top:
ret i64 0
}

julia> GPUCompiler.@override method_table child() = 1

julia> native_code_llvm(kernel, Tuple{})
; @ REPL[1]:1 within `kernel`
; Function Attrs: alwaysinline
define i64 @julia_kernel_1124() local_unnamed_addr #0 {
top:
ret i64 0
}
```

Start by running the Julia REPL reproducer using native_code_llvm and GPUCompiler.@override, comparing the generated LLVM before and after the method override. Trace how the override affects compilation and invalidation; done means native_code_llvm(kernel, Tuple{}) reflects child() returning 1 after the override.

Written by the indexing model from the issue text.