[llvm] [BPF] Handle unreachable with a kfunc call (PR #131731)

[Yingchi Long via llvm-commits]

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@@ -521,10 +526,12 @@ SDValue BPFTargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
     Callee = DAG.getTargetGlobalAddress(G->getGlobal(), CLI.DL, PtrVT,
                                         G->getOffset(), 0);
   } else if (ExternalSymbolSDNode *E = dyn_cast<ExternalSymbolSDNode>(Callee)) {
-    Callee = DAG.getTargetExternalSymbol(E->getSymbol(), PtrVT, 0);
-    fail(CLI.DL, DAG,
-         Twine("A call to built-in function '" + StringRef(E->getSymbol()) +
-               "' is not supported."));
+    if (strcmp(E->getSymbol(), BPF_UNREACHABLE) != 0) {
----------------
inclyc wrote:

```suggestion
    if (StringRef(E->getSymbol()) != BPF_UNREACHABLE) {
```

https://github.com/llvm/llvm-project/pull/131731