[Intrinsic] Signed and Unsigned Saturation Subtraction Intirnsics

Add an intrinsic that takes 2 integers and perform saturation subtraction on
them.

This is a part of implementing fixed point arithmetic in clang where some of
the more complex operations will be implemented as intrinsics.

Differential Revision: https://reviews.llvm.org/D53783

llvm-svn: 345512
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp
index 1c9a493..bae163d 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp
@@ -286,6 +286,8 @@
 
   case ISD::SADDSAT:                    return "saddsat";
   case ISD::UADDSAT:                    return "uaddsat";
+  case ISD::SSUBSAT:                    return "ssubsat";
+  case ISD::USUBSAT:                    return "usubsat";
 
   // Conversion operators.
   case ISD::SIGN_EXTEND:                return "sign_extend";