[BPF] handle opaque-pointer for __builtin_preserve_enum_value

Opaque pointer [1] is enabled as the default with commit [2].
Andrii found that current __builtin_preserve_enum_value() can only handle
non opaque pointer code pattern and will segfault with latest
llvm main branch where opaque-pointer is enabled by default.

This patch added the opaque pointer support.
Besides llvm selftests, also verified with bpf-next bpf selftests.

  [1] https://llvm.org/docs/OpaquePointers.html
  [2] https://reviews.llvm.org/D123122

Differential Revision: https://reviews.llvm.org/D123800