Implement necessary bits for flt_rounds gcc builtin. 
Codegen bits and llvm-gcc support will follow.

llvm-svn: 44182
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index d92bb9cb..38987d9 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -3739,6 +3739,7 @@
   case ISD::SIGN_EXTEND_INREG: return "sign_extend_inreg";
   case ISD::TRUNCATE:    return "truncate";
   case ISD::FP_ROUND:    return "fp_round";
+  case ISD::FLT_ROUNDS:  return "flt_rounds";
   case ISD::FP_ROUND_INREG: return "fp_round_inreg";
   case ISD::FP_EXTEND:   return "fp_extend";