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

lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -937,22 +937,6 @@ bool InstCombiner::WillNotOverflowSignedAdd(Value *LHS, Value *RHS,
   return false;
 }
 
-/// WillNotOverflowUnsignedAdd - Return true if we can prove that:
-///    (zext (add LHS, RHS))  === (add (zext LHS), (zext RHS))
-bool InstCombiner::WillNotOverflowUnsignedAdd(Value *LHS, Value *RHS,
-                                              Instruction *CxtI) {
-  // There are different heuristics we can use for this. Here is a simple one.
-  // If the sign bit of LHS and that of RHS are both zero, no unsigned wrap.
-  bool LHSKnownNonNegative, LHSKnownNegative;
-  bool RHSKnownNonNegative, RHSKnownNegative;
-  ComputeSignBit(LHS, LHSKnownNonNegative, LHSKnownNegative, /*Depth=*/0, CxtI);
-  ComputeSignBit(RHS, RHSKnownNonNegative, RHSKnownNegative, /*Depth=*/0, CxtI);
-  if (LHSKnownNonNegative && RHSKnownNonNegative)
-    return true;
-
-  return false;
-}
-
 /// \brief Return true if we can prove that:
 ///    (sub LHS, RHS)  === (sub nsw LHS, RHS)
 /// This basically requires proving that the add in the original type would not
@@ -1327,7 +1311,9 @@ Instruction *InstCombiner::visitAdd(BinaryOperator &I) {
     Changed = true;
     I.setHasNoSignedWrap(true);
   }
-  if (!I.hasNoUnsignedWrap() && WillNotOverflowUnsignedAdd(LHS, RHS, &I)) {
+  if (!I.hasNoUnsignedWrap() &&
+      computeOverflowForUnsignedAdd(LHS, RHS, &I) ==
+          OverflowResult::NeverOverflows) {
     Changed = true;
     I.setHasNoUnsignedWrap(true);
   }