[llvm] [ConstantTime][LLVM] Add llvm.ct.select intrinsic with generic SelectionDAG lowering (PR #166702)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> @nikic Huh, makes sense, thanks! Though it may as well be abstracted to "a qualifier to the existing instruction", like the fast maths flags.

Fast math flags are another droppable annotation 

https://github.com/llvm/llvm-project/pull/166702