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

[Folkert de Vries via llvm-commits]

folkertdev wrote:

Allright, so generally you'd like to see an x86 implementation that resembles the arm one more? Because currently the x86 implementation is quite different (and, as established, very broken).

This will be tough (for me) but I'm willing to give that a go.

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