[llvm] [SDAG] Fix llvm.modf for ppc_fp128 (attempt two) (PR #127976)

[Benjamin Maxwell via llvm-commits]

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@@ -2644,8 +2644,10 @@ bool SelectionDAG::expandMultipleResultFPLibCall(
     // optimized out. This prevents an FP stack pop from being emitted for it.
     // Setting the root like this ensures there will be a use of the
     // `CopyFromReg` chain, and ensures the FP pop will be emitted.
+    SDValue OldRoot = getRoot();
     SDValue NewRoot =
-        getNode(ISD::TokenFactor, DL, MVT::Other, getRoot(), CallChain);
+        OldRoot ? getNode(ISD::TokenFactor, DL, MVT::Other, OldRoot, CallChain)
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MacDue wrote:

Ugh, it's because of:
https://github.com/llvm/llvm-project/blob/dc326d0b01f63e49f4f11c0c332369bf109721df/llvm/lib/CodeGen/SelectionDAG/LegalizeTypes.cpp#L209-L211

Which then later ignores any changes to the root:
https://github.com/llvm/llvm-project/blob/dc326d0b01f63e49f4f11c0c332369bf109721df/llvm/lib/CodeGen/SelectionDAG/LegalizeTypes.cpp#L438-L439

Which I think would be incorrect for the workround the multiple-result expansions are doing. 

https://github.com/llvm/llvm-project/pull/127976