[llvm] [GlobalISel] Introduce `G_POISON` (PR #127825)

[Mateusz Sokół via llvm-commits]

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@@ -8451,7 +8455,8 @@ LegalizerHelper::lowerVECTOR_COMPRESS(llvm::MachineInstr &MI) {
   auto OutPos = MIRBuilder.buildConstant(IdxTy, 0);
 
   bool HasPassthru =
-      MRI.getVRegDef(Passthru)->getOpcode() != TargetOpcode::G_IMPLICIT_DEF;
+      MRI.getVRegDef(Passthru)->getOpcode() != TargetOpcode::G_IMPLICIT_DEF &&
+      MRI.getVRegDef(Passthru)->getOpcode() != TargetOpcode::G_POISON;
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mtsokol wrote:

Done!

https://github.com/llvm/llvm-project/pull/127825