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

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw requested changes to this pull request.

A correctness issue was identified by [llvm-hackme](https://github.com/dtcxzyw/llvm-hackme). Please see the [issue comment](https://github.com/llvm/llvm-project/pull/166702#issuecomment-5018422582) for details.

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