Fix typo in step intrinsic comment (#153642)

`y` should be the first argument and `x` should be the second, otherwise
the formula is wrong. This also matches the documentation
[here](https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-step).

Author: kmpeng

clang/lib/Headers/hlsl/hlsl_alias_intrinsics.h

@@ -2190,10 +2190,10 @@ float4 sqrt(float4);
 // step builtins
 //===----------------------------------------------------------------------===//
 
-/// \fn T step(T x, T y)
+/// \fn T step(T y, T x)
 /// \brief Returns 1 if the x parameter is greater than or equal to the y
-/// parameter; otherwise, 0. vector. \param x [in] The first floating-point
-/// value to compare. \param y [in] The first floating-point value to compare.
+/// parameter; otherwise, 0. vector. \param y [in] The first floating-point
+/// value to compare. \param x [in] The second floating-point value to compare.
 ///
 /// Step is based on the following formula: (x >= y) ? 1 : 0