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

[Akshay K via llvm-commits]

================

----------------
kumarak wrote:

Thanks, @arsenm, for bringing it up. We were not sure where the fallback approach should be handled. Having it inside DAGBuilder blindsided us into creating an illegal type that was cleaned up downstream. Moved it to the legalizer, and for types that are not legalized (such as f64 on i386 without SSE or f128, etc.), it uses memory backing. 

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