[llvm] Add known and demanded bits support for zext nneg (PR #70858)

[via llvm-commits]

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@@ -1103,6 +1103,9 @@ static void computeKnownBitsFromOperator(const Operator *I,
     assert(SrcBitWidth && "SrcBitWidth can't be zero");
     Known = Known.anyextOrTrunc(SrcBitWidth);
     computeKnownBits(I->getOperand(0), Known, Depth + 1, Q);
+    if (auto *Inst = dyn_cast<PossiblyNonNegInst>(I);
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goldsteinn wrote:

Guess the syntax `if (A = expr; other_stuff...)` is a pretty abnormal pattern and imo prone to be misread, but its a nit and nikics approved so no need to change if you feel otherwise.

https://github.com/llvm/llvm-project/pull/70858