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

[Benjamin Maxwell via llvm-commits]

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@@ -435,8 +435,21 @@ bool DAGTypeLegalizer::run() {
 #endif
     PerformExpensiveChecks();
 
-  // If the root changed (e.g. it was a dead load) update the root.
-  DAG.setRoot(Dummy.getValue());
+  // Get the value of the original root after type legalization.
+  SDValue Root = Dummy.getValue();
+
+  // Get the current root value, if it's not null combine it with the original
+  // root to prevent it being removed as a dead node.
+  if (SDValue LegalRoot = DAG.getRoot()) {
+    Root = DAG.getNode(ISD::TokenFactor, SDLoc(LegalRoot), MVT::Other, Root,
+                       LegalRoot);
+    // The token_factor should not need any legalization (as both inputs have
+    // already been legalized).
+    Root->setNodeId(Processed);
+  }
+
+  // Restore the root.
+  DAG.setRoot(Root);
----------------
MacDue wrote:

Any thoughts? I can't think of an obvious way to avoid the need of the workaround for the expansion of `modf` and `frexp`, and I don't think it's safe to do if LegalizeTypes types just blindly ignores any changes to the root. 

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