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

[Patrick Simmons via llvm-commits]

================
@@ -0,0 +1,261 @@
+#pragma once
----------------
linuxrocks123 wrote:

@arsenm this code will need to be used by multiple passes, so it belongs in a header.

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