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

[via llvm-commits]

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@@ -242,11 +242,15 @@ class LLVM_ABI TargetLoweringBase {
 
   /// Enum that describes what type of support for selects the target has.
   enum SelectSupportKind {
-    ScalarValSelect,      // The target supports scalar selects (ex: cmov).
-    ScalarCondVectorVal,  // The target supports selects with a scalar condition
-                          // and vector values (ex: cmov).
-    VectorMaskSelect      // The target supports vector selects with a vector
-                          // mask (ex: x86 blends).
+    ScalarValSelect,     // The target supports scalar selects (ex: cmov).
+    ScalarCondVectorVal, // The target supports selects with a scalar condition
+                         // and vector values (ex: cmov).
+    VectorMaskSelect,    // The target supports vector selects with a vector
+                         // mask (ex: x86 blends).
+    CtSelect,            // The target implements a custom constant-time select.
+    ScalarCondVectorValCtSelect, // The target supports selects with a scalar
+                                 // condition and vector values.
+    VectorMaskValCtSelect, // The target supports vector selects with a vector
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kumarak wrote:

We only rely on `CtSelect` to decide target lowering support for the `CTSELECT` dag node. Other two should be removed here. If needed for supporting other targets in future, we can introduce them again. 

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