GlobalISel: Use default rounding mode when extending fconstant

[arsenm]

Summary

I don't think this matters since the values should all be exactly representable.

[rtereshin]

I don't think this matters since the values should all be exactly representable.

We could probably assert LosesInfo being false in here.

[arsenm]

Add assert

[arsenm]

r353844

lib/CodeGen/GlobalISel/LegalizerHelper.cpp

LegalizerHelper::widenScalar(MachineInstr &MI, unsigned TypeIdx, LLT WideTy) {
   }
   case TargetOpcode::G_FCONSTANT: {
     MachineOperand &SrcMO = MI.getOperand(1);
     LLVMContext &Ctx = MIRBuilder.getMF().getFunction().getContext();
     APFloat Val = SrcMO.getFPImm()->getValueAPF();
     bool LosesInfo;
     switch (WideTy.getSizeInBits()) {
     case 32:
-      Val.convert(APFloat::IEEEsingle(), APFloat::rmTowardZero, &LosesInfo);
+      Val.convert(APFloat::IEEEsingle(), APFloat::rmNearestTiesToEven,
+                  &LosesInfo);
       break;
     case 64:
-      Val.convert(APFloat::IEEEdouble(), APFloat::rmTowardZero, &LosesInfo);
+      Val.convert(APFloat::IEEEdouble(), APFloat::rmNearestTiesToEven,
+                  &LosesInfo);
       break;
     default:
-      llvm_unreachable("Unhandled fp widen type");
+      return UnableToLegalize;
     }
+
+    assert(!LosesInfo && "extend should always be lossless");
+
     Observer.changingInstr(MI);
     SrcMO.setFPImm(ConstantFP::get(Ctx, Val));
 
     widenScalarDst(MI, WideTy, 0, TargetOpcode::G_FPTRUNC);
     Observer.changedInstr(MI);
     return Legalized;
   }
   case TargetOpcode::G_IMPLICIT_DEF: {