[clang] [llvm] [SPIRV] Add PreLegalizer pattern matching for `faceforward` (PR #139959)

[Farzon Lotfi via cfe-commits]

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@@ -58,3 +58,106 @@ void SPIRVCombinerHelper::applySPIRVDistance(MachineInstr &MI) const {
 
   MI.eraseFromParent();
 }
+
+/// This match is part of a combine that
+/// rewrites select(fcmp(dot(I, Ng), 0), N, -N) to faceforward(N, I, Ng)
+///   (vXf32 (g_select
+///             (g_fcmp
+///                (g_intrinsic dot(vXf32 I) (vXf32 Ng)
+///                 0)
+///             (vXf32 N)
+///             (vXf32 g_fneg (vXf32 N))))
+/// ->
+///   (vXf32 (g_intrinsic faceforward
+///             (vXf32 N) (vXf32 I) (vXf32 Ng)))
+///
+/// This only works for Vulkan targets.
----------------
farzonl wrote:

```suggestion
/// This only works for Vulkan shader targets.
```

https://github.com/llvm/llvm-project/pull/139959