[llvm] [Bitcode] Fix constexpr expansion creating invalid PHIs (PR #141560)

[Nikita Popov via llvm-commits]

================
@@ -6095,14 +6095,18 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
         // seen value here, to avoid expanding a constant expression multiple
         // times.
         auto It = Args.find(BB);
+        BasicBlock *EdgeBB = ConstExprEdgeBBs.lookup({BB, CurBB});
         if (It != Args.end()) {
-          PN->addIncoming(It->second, BB);
+          /// If this predecessor was also replaced with a constexpr basic
+          /// block, it must be de-duplicated.
+          if (!EdgeBB) {
+            PN->addIncoming(It->second, BB);
+          }
----------------
nikic wrote:

```suggestion
          // If this predecessor was also replaced with a constexpr basic
          // block, it must be de-duplicated.
          if (!EdgeBB)
            PN->addIncoming(It->second, BB);
```

https://github.com/llvm/llvm-project/pull/141560