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

[Julius Alexandre via llvm-commits]

================
@@ -13209,6 +13213,67 @@ SDValue DAGCombiner::visitSELECT(SDNode *N) {
   return SDValue();
 }
 
+// Keep CT_SELECT combines deliberately conservative to preserve constant-time
+// intent across generic DAG combines. We only accept:
+//  - canonicalization of negated conditions (flip true/false operands), and
+//  - i1 CT_SELECT nesting merges via AND/OR that keep the result as CT_SELECT.
+// Broader rewrites should be done in target-specific lowering when stronger
+// guarantees about legality and constant-time preservation are available.
+SDValue DAGCombiner::visitCT_SELECT(SDNode *N) {
+  SDValue N0 = N->getOperand(0);
+  SDValue N1 = N->getOperand(1);
+  SDValue N2 = N->getOperand(2);
+  EVT VT = N->getValueType(0);
+  EVT VT0 = N0.getValueType();
+  SDLoc DL(N);
+  SDNodeFlags Flags = N->getFlags();
----------------
wizardengineer wrote:

None. CT_SELECT no longer carries flags, so I removed the flag handling in 64613ed31ef6.

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