[llvm] [InstCombine] Fix assertion failure in issue80597 (PR #80614)

[Nikita Popov via llvm-commits]

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@@ -801,6 +802,10 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
         LShr->takeName(I);
         return InsertNewInstWith(LShr, I->getIterator());
       } else if (Known.One[BitWidth-ShiftAmt-1]) { // New bits are known one.
+        // Sync SignBits with computeKnownBits to make sure there is no
+        // conflict.
+        HighBits = APInt::getHighBitsSet(
+            BitWidth, std::min(MinSignBits + ShiftAmt - 1, BitWidth));
         Known.One |= HighBits;
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nikic wrote:

Can we do `Known.Zero &= ~HighBits;` instead?

https://github.com/llvm/llvm-project/pull/80614