[llvm] Fix APInt::concat for zero-width operands (PR #207475)

[Nikita Popov via llvm-commits]

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@@ -952,6 +952,10 @@ class [[nodiscard]] APInt {
   /// equivalent to:
   ///   (this->zext(NewWidth) << NewLSB.getBitWidth()) | NewLSB.zext(NewWidth)
   APInt concat(const APInt &NewLSB) const {
+    if (getBitWidth() == 0)
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nikic wrote:

Only this check would be sufficient to avoid UB, right?

https://github.com/llvm/llvm-project/pull/207475