[llvm] [BPI] Transfer value-handles when assign/move constructing BPI (PR #77774)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> (No test because IMHO testing for a behaviour that varies with memory allocators is likely futile; I can add the reproducer with a CHECK for the relevant branch weights if it's desired though)

Agree that many times a test is not viable. Certain issues can be detected using a `LLVM_ENABLE_REVERSE_ITERATION=on` build.

We downstream check whether stage 2 and stage 3 clang executables are the same. This is a very heavy test.

Another way to check certain non-determinism problems is to replace the allocator.

https://github.com/llvm/llvm-project/pull/77774