[Hexagon] Return 0 from getDotNewPredOp when .new opcode does not exist

This allows using this function to test if an instruction can be converted
to a .new form.

llvm-svn: 304549
diff --git a/llvm/lib/Target/Hexagon/HexagonInstrInfo.cpp b/llvm/lib/Target/Hexagon/HexagonInstrInfo.cpp
index 0fef91e..b76da72 100644
--- a/llvm/lib/Target/Hexagon/HexagonInstrInfo.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonInstrInfo.cpp
@@ -3419,9 +3419,7 @@
   int NewOpcode = Hexagon::getPredNewOpcode(MI.getOpcode());
   if (NewOpcode >= 0)
     return NewOpcode;
-
-  dbgs() << "Cannot convert to .new: " << getName(MI.getOpcode()) << '\n';
-  llvm_unreachable(nullptr);
+  return 0;
 }
 
 int HexagonInstrInfo::getDotOldOp(const MachineInstr &MI) const {