[llvm] [DAGCombiner] Fold sext_in_reg when adding sign bit to an already extended value. (PR #214848)

[Matt Arsenault via llvm-commits]

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arsenm wrote:

By m_Deferred did you mean m_SpecificVal? I'd like to see dedicated negative tests for mismatched value, and mismatched bit counts for the below check 

https://github.com/llvm/llvm-project/pull/214848