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

[Matt Arsenault via llvm-commits]

================
@@ -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)
----------------
arsenm wrote:

Why is this null here? 

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