InstCombine: sub nsw %x, C -> add nsw %x, -C if C isn't INT_MIN

We can preserve nsw during this transform if -C won't overflow.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216269 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Constant.h

@@ -69,6 +69,9 @@ public:
   /// Return true if the value is negative zero or null value.
   bool isZeroValue() const;
 
+  /// \brief Return true if the value is not the smallest signed value.
+  bool isNotMinSignedValue() const;
+
   /// \brief Return true if the value is the smallest signed value.
   bool isMinSignedValue() const;