[JITLink] Allow pre-existing eh-frame CIE edges on FDEs.

This restores the pre-b9383a86b8f behavior. Most platforms / compilers don't
add relocations for CIEs, however they're not prohibited and we want objects
that contain them to remain loadable.

Author: lhames

llvm/lib/ExecutionEngine/JITLink/EHFrameSupport.cpp

@@ -304,24 +304,39 @@ Error EHFrameEdgeFixer::processFDE(ParseContext &PC, Block &B,
   {
     // Process the CIE pointer field.
     auto CIEEdgeItr = BlockEdges.find(CIEDeltaFieldOffset);
-    if (CIEEdgeItr != BlockEdges.end())
-      return make_error<JITLinkError>(
-          "CIE pointer field already has edge at " +
-          formatv("{0:x16}", RecordAddress + CIEDeltaFieldOffset));
 
     orc::ExecutorAddr CIEAddress =
         RecordAddress + orc::ExecutorAddrDiff(CIEDeltaFieldOffset) -
         orc::ExecutorAddrDiff(CIEDelta);
-    LLVM_DEBUG({
-      dbgs() << "      Adding edge at " << (RecordAddress + CIEDeltaFieldOffset)
-             << " to CIE at: " << CIEAddress << "\n";
-    });
-    if (auto CIEInfoOrErr = PC.findCIEInfo(CIEAddress))
-      CIEInfo = *CIEInfoOrErr;
-    else
-      return CIEInfoOrErr.takeError();
-    assert(CIEInfo->CIESymbol && "CIEInfo has no CIE symbol set");
-    B.addEdge(NegDelta32, CIEDeltaFieldOffset, *CIEInfo->CIESymbol, 0);
+    if (CIEEdgeItr == BlockEdges.end()) {
+      LLVM_DEBUG({
+        dbgs() << "        Adding edge at "
+               << (RecordAddress + CIEDeltaFieldOffset)
+               << " to CIE at: " << CIEAddress << "\n";
+      });
+      if (auto CIEInfoOrErr = PC.findCIEInfo(CIEAddress))
+        CIEInfo = *CIEInfoOrErr;
+      else
+        return CIEInfoOrErr.takeError();
+      assert(CIEInfo->CIESymbol && "CIEInfo has no CIE symbol set");
+      B.addEdge(NegDelta32, CIEDeltaFieldOffset, *CIEInfo->CIESymbol, 0);
+    } else {
+      LLVM_DEBUG({
+        dbgs() << "        Already has edge at "
+               << (RecordAddress + CIEDeltaFieldOffset) << " to CIE at "
+               << CIEAddress << "\n";
+      });
+      auto &EI = CIEEdgeItr->second;
+      if (EI.Addend)
+        return make_error<JITLinkError>(
+            "CIE edge at " +
+            formatv("{0:x16}", RecordAddress + CIEDeltaFieldOffset) +
+            " has non-zero addend");
+      if (auto CIEInfoOrErr = PC.findCIEInfo(EI.Target->getAddress()))
+        CIEInfo = *CIEInfoOrErr;
+      else
+        return CIEInfoOrErr.takeError();
+    }
   }
 
   // Process the PC-Begin field.