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

[Kaitlin Peng via cfe-commits]

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@@ -0,0 +1,63 @@
+# RUN: llc -verify-machineinstrs -O0 -mtriple spirv-unknown-unknown -run-pass=spirv-prelegalizer-combiner %s -o - | FileCheck %s
+# REQUIRES: asserts
+---
+name:            faceforward_instcombine_float
+tracksRegLiveness: true
+legalized: true
+body:             |
+  bb.1.entry:
+    ; CHECK-LABEL: name: faceforward_instcombine_float
+    ; CHECK-NOT: %9:_(s32) = G_FCONSTANT float 0.000000e+00
+    ; CHECK-NOT: %8:_(s32) = G_FMUL %1:fid, %2:fid
+    ; CHECK-NOT: %10:_(s1) = G_FCMP floatpred(olt), %8:_(s32), %9:_
+    ; CHECK-NOT: %11:_(s32) = G_FNEG %0:fid
+    ; CHECK-NOT: %12:id(s32) = G_SELECT %10:_(s1), %0:fid, %11:_
+    ; CHECK: %10:id(s32) = G_INTRINSIC intrinsic(@llvm.spv.faceforward), %2(s32), %3(s32), %4(s32)
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kmpeng wrote:

I'm not really familiar with what `G_INTRINSIC_W_SIDE_EFFECTS` is. From my understanding, the spirv faceforward intrinsic is treated as a pure mathematical operation that doesn't have side effects (as it has the IntrNoMem property), so this shouldn't be needed?

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