[llvm] x86: fix musttail sibcall miscompilation (PR #168956)

[Reid Kleckner via llvm-commits]

rnk wrote:

Thanks for looking at this! I've encountered this issue before in Google's internal tracker, and if I dig through github issues, I might be able to find it. I probably don't have time to review it today, but I'll prioritize it soon. I think this other [pending PR](https://github.com/llvm/llvm-project/pull/150157) might be related.

https://github.com/llvm/llvm-project/pull/168956