[llvm] [DAGCombiner] Share byte provider steps with AMDGPU target (PR #221959)

[Matt Arsenault via llvm-commits]

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@@ -9734,24 +9733,19 @@ calculateByteProvider(SDValue Op, unsigned Index, unsigned Depth,
     return std::nullopt;
   unsigned ByteWidth = BitWidth / 8;
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arsenm wrote:

Or just if (condition) llvm_unreachable 

https://github.com/llvm/llvm-project/pull/221959