InstCombine: Fix a potential bug in 0 - (X sdiv C) -> (X sdiv -C)

While *most* (X sdiv 1) operations will get caught by InstSimplify, it
is still possible for a sdiv to appear in the worklist which hasn't been
simplified yet.

This means that it is possible for 0 - (X sdiv 1) to get transformed
into (X sdiv -1); dividing by -1 can make the transform produce undef
values instead of the proper result.

Sorry for the lack of testcase, it's a bit problematic because it relies
on the exact order of operations in the worklist.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215818 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/Constant.h

@@ -53,6 +53,9 @@ public:
   /// getNullValue.
   bool isNullValue() const;
 
+  /// \brief Returns true if the value is one.
+  bool isOneValue() const;
+
   /// isAllOnesValue - Return true if this is the value that would be returned by
   /// getAllOnesValue.
   bool isAllOnesValue() const;