[llvm] [x86] Enable indirect tail calls with more arguments (PR #137643)

[Hans Wennborg via llvm-commits]

zmodem wrote:

Sorry for dropping the ball here. I've kept the tab open with the intention of coming back and fixing this properly, but obviously that didn't happen yet.

The way I remember it, I kept finding more and more questionable things (like `X86tcret_6regs` and `X86tcret_1reg`), which I wanted to fix properly, and then of course the hard part becomes writing tests for it.

I still want to come back to this (maybe a project for the holidays :), but if someone wants to take over the PR and run with it, that's fine too.

https://github.com/llvm/llvm-project/pull/137643