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

[via llvm-commits]

================
@@ -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).
----------------
goldsteinn wrote:

Ill port

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