[SelectionDAG] Remove UnsafeFPMath in LegalizeDAG (#146316)

These global flags hinder further improvements like [[RFC] Honor pragmas
with
-ffp-contract=fast](https://discourse.llvm.org/t/rfc-honor-pragmas-with-ffp-contract-fast)
and pass concurrency support. Remove them incrementally.

Author: paperchalice

llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

@@ -3853,7 +3853,7 @@ bool SelectionDAGLegalize::ExpandNode(SDNode *Node) {
     break;
   case ISD::FP_TO_FP16:
     LLVM_DEBUG(dbgs() << "Legalizing FP_TO_FP16\n");
-    if (!TLI.useSoftFloat() && TM.Options.UnsafeFPMath) {
+    if (Node->getFlags().hasApproximateFuncs() && !TLI.useSoftFloat()) {
       SDValue Op = Node->getOperand(0);
       MVT SVT = Op.getSimpleValueType();
       if ((SVT == MVT::f64 || SVT == MVT::f80) &&

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -3923,11 +3923,15 @@ void SelectionDAGBuilder::visitFPTrunc(const User &I) {
   // FPTrunc is never a no-op cast, no need to check
   SDValue N = getValue(I.getOperand(0));
   SDLoc dl = getCurSDLoc();
+  SDNodeFlags Flags;
+  if (auto *TruncInst = dyn_cast<FPMathOperator>(&I))
+    Flags.copyFMF(*TruncInst);
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
   EVT DestVT = TLI.getValueType(DAG.getDataLayout(), I.getType());
   setValue(&I, DAG.getNode(ISD::FP_ROUND, dl, DestVT, N,
                            DAG.getTargetConstant(
-                               0, dl, TLI.getPointerTy(DAG.getDataLayout()))));
+                               0, dl, TLI.getPointerTy(DAG.getDataLayout())),
+                           Flags));
 }
 
 void SelectionDAGBuilder::visitFPExt(const User &I) {