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

[Julius Alexandre via llvm-commits]

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wizardengineer wrote:

The two safe folds, a literal constant condition and identical arms, are done in InstructionSimplify for ct_select. f4383a332170 notes those are the only folds LangRef allows, so we do not fold through known bits and lose the constant-time property.

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