Analysis: Reformulate WillNotOverflowUnsignedAdd for reusability

WillNotOverflowUnsignedAdd's smarts will live in ValueTracking as
computeOverflowForUnsignedAdd.  It now returns a tri-state result:
never overflows, always overflows and sometimes overflows.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225329 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/ValueTracking.h

@@ -221,6 +221,11 @@ namespace llvm {
                                                AssumptionCache *AC,
                                                const Instruction *CxtI,
                                                const DominatorTree *DT);
+  OverflowResult computeOverflowForUnsignedAdd(Value *LHS, Value *RHS,
+                                               const DataLayout *DL,
+                                               AssumptionCache *AC,
+                                               const Instruction *CxtI,
+                                               const DominatorTree *DT);
 } // end namespace llvm
 
 #endif