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

[Patrick Simmons via llvm-commits]

linuxrocks123 wrote:

@jayfoad this is part of a project to write a "perfect" register allocator that allocates directly out of SSA form.  Since there will no longer be any "after SSA but before register allocation" IR state once this project is completed, it is necessary to move up certain passes that currently run before register allocation but after transformation out of SSA form so that those passes run in SSA form instead.

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