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

[Kaitlin Peng via cfe-commits]

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@@ -95,21 +108,132 @@ void applySPIRVDistance(MachineInstr &MI, MachineRegisterInfo &MRI,
 
   SPIRVGlobalRegistry *GR =
       MI.getMF()->getSubtarget<SPIRVSubtarget>().getSPIRVGlobalRegistry();
-  auto RemoveAllUses = [&](Register Reg) {
-    SmallVector<MachineInstr *, 4> UsesToErase(
-        llvm::make_pointer_range(MRI.use_instructions(Reg)));
-
-    // calling eraseFromParent to early invalidates the iterator.
-    for (auto *MIToErase : UsesToErase) {
-      GR->invalidateMachineInstr(MIToErase);
-      MIToErase->eraseFromParent();
-    }
-  };
-  RemoveAllUses(SubDestReg);   // remove all uses of FSUB Result
+  removeAllUses(SubDestReg, MRI, GR); // remove all uses of FSUB Result
   GR->invalidateMachineInstr(SubInstr);
   SubInstr->eraseFromParent(); // remove FSUB instruction
 }
 
+/// 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.
+///
+bool matchSelectToFaceForward(MachineInstr &MI, MachineRegisterInfo &MRI) {
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kmpeng wrote:

I created a new PR to add the combiner helper and move the current length to distance pattern matching code into it, could you take a look? https://github.com/llvm/llvm-project/pull/162735 
Looking to get that in before continuing with this PR.

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