[llvm] Move SI Lower Control Flow Up (PR #159557)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> The author prefers to keep working this ticket, please dont close it.

This is not up to the author. I am declaring this PR unreviewable; do not reopen it. Any further changes in this area need to be a new PR, not cluttered with a deeply flawed started point and this many comments 

https://github.com/llvm/llvm-project/pull/159557