[clang] [llvm] [HLSL] Implement `WaveReadLaneAt` intrinsic (PR #111010)

[Farzon Lotfi via cfe-commits]

================
@@ -18835,6 +18835,24 @@ case Builtin::BI__builtin_hlsl_elementwise_isinf: {
     Intrinsic::ID ID = CGM.getHLSLRuntime().getWaveIsFirstLaneIntrinsic();
     return EmitRuntimeCall(Intrinsic::getDeclaration(&CGM.getModule(), ID));
   }
+  case Builtin::BI__builtin_hlsl_wave_read_lane_at: {
+    // Due to the use of variadic arguments we must explicitly retreive them and
+    // create our function type.
+    Value *OpExpr = EmitScalarExpr(E->getArg(0));
+    Value *OpIndex = EmitScalarExpr(E->getArg(1));
+    llvm::FunctionType *FT = llvm::FunctionType::get(
+        OpExpr->getType(), ArrayRef{OpExpr->getType(), OpIndex->getType()},
+        false);
+
+    // Get overloaded name
+    std::string name =
+        Intrinsic::getName(CGM.getHLSLRuntime().getWaveReadLaneAtIntrinsic(),
+                           ArrayRef{OpExpr->getType()}, &CGM.getModule());
+    return EmitRuntimeCall(CGM.CreateRuntimeFunction(FT, name, {},
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farzonl wrote:

It will  we need to modify https://github.com/llvm/llvm-project/blob/848cb21ddca5dc4d8c52e487264936be8f1bfe8d/llvm/lib/Transforms/Scalar/Scalarizer.cpp#L748

to become:
```c++
if (isVectorIntrinsicWithScalarOpAtArg(ID, J) || TTI->isTargetIntrinsicWithScalarOpAtArg(ID, J)) 
```
by adding a new `TargetTransformInfo` api

https://github.com/llvm/llvm-project/pull/111010