[llvm] [LoongArch] Use fallback path when relocation `SymA == nullptr` (PR #196940)

[via llvm-commits]

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@@ -403,6 +403,9 @@ bool LoongArchAsmBackend::addReloc(const MCFragment &F, const MCFixup &Fixup,
   };
   uint64_t FixedValueA, FixedValueB;
   if (Target.getSubSym()) {
+    if (!Target.getAddSym())
+      return Fallback();
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wangleiat wrote:

A short comment would be helpful, but otherwise LGTM.

https://github.com/llvm/llvm-project/pull/196940