[llvm] [X86] Improve helper for simplifying demanded bits of compares (PR #84360)

[via llvm-commits]

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@@ -41293,6 +41293,154 @@ static SDValue combineShuffle(SDNode *N, SelectionDAG &DAG,
   return SDValue();
 }
 
+// Simplify a decomposed (sext (setcc)). Assumes prior check that
+// bitwidth(sext)==bitwidth(setcc operands).
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goldsteinn wrote:

So there where no test changes outside of X86.

I would prefer to keep in x86 for now, im a bit worried about whether the float semantics will translate to other targets.
Ill leave a note in simplfyDemandedBits that if we want to extend `ISD::SETCC` it may be worth porting.

https://github.com/llvm/llvm-project/pull/84360