codegen: simplify GC pointer zeroing in value_to_pointer
Use Julia's datatype layout information (jl_ptr_offset) to iterate
through pointer fields instead of walking LLVM types with
TrackCompositeType. This approach is simpler and consistent with
similar code like undef_derived_strct.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
