[llvm] [GlobalISel] Drop poison flags by default in combiners (PR #218306)

[Matt Arsenault via llvm-commits]

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@@ -739,6 +739,11 @@ class GIMatchTableExecutor {
     llvm_unreachable("Subclass does not implement runCustomAction!");
   }
 
+  /// Return true to drop root poison-generating flags from the implicit flag
+  /// propagation performed for newly-built instructions. Explicit MIFlags
+  /// actions in the match table still apply to the output instruction.
+  virtual bool shouldDropRootPoisonGeneratingFlags() const { return false; }
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arsenm wrote:

This control doesn't make sense, and has no uses. This is not a property of a match table, but a specific pattern. There shouldn't be a virtual for anything related to this 

https://github.com/llvm/llvm-project/pull/218306