[llvm] [DirectX] Add support for vector_reduce_add (PR #117646)

[Farzon Lotfi via llvm-commits]

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@@ -67,11 +67,44 @@ static bool isIntrinsicExpansion(Function &F) {
   case Intrinsic::dx_sign:
   case Intrinsic::dx_step:
   case Intrinsic::dx_radians:
+  case Intrinsic::vector_reduce_add:
+  case Intrinsic::vector_reduce_fadd:
     return true;
   }
   return false;
 }
 
+static Value *expandVecReduceFAdd(CallInst *Orig) {
+  // Note: vector_reduce_fadd first argument is a starting value
+  // Our use doesn't need it, so ignoring argument zero.
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farzonl wrote:

This behavior is tested via tests with the  `*_start1` postfix.

https://github.com/llvm/llvm-project/pull/117646