[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:

I think this will have to be updated to preserve the root created during legalize types, as otherwise the workaround needed for functions like `modf` and `frexp` when only the result from the output pointer is used won't work (in all cases),  and I can't think of any obvious alternate solution here. 

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