[clang] [llvm] [HLSL] Implement the `smoothstep` intrinsic (PR #132288)

[Farzon Lotfi via cfe-commits]

================
@@ -20955,6 +20955,25 @@ Value *CodeGenFunction::EmitSPIRVBuiltinExpr(unsigned 
BuiltinID,
         /*ReturnType=*/I->getType(), Intrinsic::spv_reflect,
         ArrayRef<Value *>{I, N}, nullptr, "spv.reflect");
   }
+  case SPIRV::BI__builtin_spirv_smoothstep: {
+    Value *Min = EmitScalarExpr(E->getArg(0));
+    Value *Max = EmitScalarExpr(E->getArg(1));
+    Value *X = EmitScalarExpr(E->getArg(2));
+    assert(E->getArg(0)->getType()->hasFloatingRepresentation() &&
+           E->getArg(1)->getType()->hasFloatingRepresentation() &&
+           E->getArg(2)->getType()->hasFloatingRepresentation() &&
+           "SmoothStep operands must have a float representation");
+    assert((E->getArg(0)->getType()->isScalarType() ||
+            E->getArg(0)->getType()->isVectorType()) &&
+           (E->getArg(1)->getType()->isScalarType() ||
+            E->getArg(1)->getType()->isVectorType()) &&
+           (E->getArg(2)->getType()->isScalarType() ||
+            E->getArg(2)->getType()->isVectorType()) &&
+           "SmoothStep operands must be a scalar or vector");
+    return Builder.CreateIntrinsic(
+        /*ReturnType=*/Min->getType(), Intrinsic::spv_smoothstep,
+        ArrayRef<Value *>{Min, Max, X}, nullptr, "spv.smoothstep");
----------------
farzonl wrote:

could you put an inline comment for the nullptr so I know what exactly we are 
setting to nullptr?
```cpp
/*blah=*/nullptr
```

https://github.com/llvm/llvm-project/pull/132288
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