[llvm] [BPF] report `Invalid usage of the XADD return value"` elegantly (PR #92742)

[via llvm-commits]

eddyz87 wrote:

The doc for `diagnose` says:

> This function returns, ..., so the caller should leave the compilation process in a self-consistent state, even though the generated code need not be correct.

In this case, we will have a register used in some later instructions, this does not seem to violate any IR-level constraints, so this change should be fine as far as I understand.

https://github.com/llvm/llvm-project/pull/92742