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

[Matt Arsenault via llvm-commits]

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

llvm does't use pragma once. Also missing file header.

There's also way too much code in the header, sink everything to the implementation file 

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