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

[Marvin Häuser via llvm-commits]

mhaeuser 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.

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