[LegalizeTypes][RISCV][X86] Legalize FP_ROUND to libcall in SoftPromoteHalfRes_FP_ROUND if the input type is softened. (llvm#119481)

Previously we created an FP_TO_FP16 and legalized it in
SoftenFloatOp_FP_ROUND. This caused i16 to be sent to call lowering
instead of f16. This results in the ABI not being followed if f16 is
supposed to be passed in a different register than i16.

Looking at the libgcc binary for the library function it appears the value
is returned in xmm0 so the X86 test was being miscompiled before.

Fixes llvm#107607.

Author: topperc

llvm/lib/CodeGen/SelectionDAG/LegalizeFloatTypes.cpp

@@ -3415,6 +3415,23 @@ SDValue DAGTypeLegalizer::SoftPromoteHalfRes_FP_ROUND(SDNode *N) {
   SDValue Op = N->getOperand(IsStrict ? 1 : 0);
   EVT SVT = Op.getValueType();
 
+  // If the input type needs to be softened, do that now so that call lowering
+  // will see the f16 type.
+  if (getTypeAction(SVT) == TargetLowering::TypeSoftenFloat) {
+    RTLIB::Libcall LC = RTLIB::getFPROUND(SVT, RVT);
+    assert(LC != RTLIB::UNKNOWN_LIBCALL && "Unsupported FP_ROUND libcall");
+
+    SDValue Chain = IsStrict ? N->getOperand(0) : SDValue();
+    Op = GetSoftenedFloat(Op);
+    TargetLowering::MakeLibCallOptions CallOptions;
+    CallOptions.setTypeListBeforeSoften(SVT, RVT, true);
+    std::pair<SDValue, SDValue> Tmp =
+        TLI.makeLibCall(DAG, LC, RVT, Op, CallOptions, SDLoc(N), Chain);
+    if (IsStrict)
+      ReplaceValueWith(SDValue(N, 1), Tmp.second);
+    return DAG.getNode(ISD::BITCAST, SDLoc(N), MVT::i16, Tmp.first);
+  }
+
   if (IsStrict) {
     SDValue Res = DAG.getNode(GetPromotionOpcodeStrict(SVT, RVT), SDLoc(N),
                               {MVT::i16, MVT::Other}, {N->getOperand(0), Op});