[ValueTracking] Add logic for isKnownNonZero(sadd.sat X, Y)

The logic here is the same for `add` so reuse the existing helper
`isNonZeroAdd`

Alive2 Link:
    https://alive2.llvm.org/ce/z/mhKvC5

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D149414

Author: goldsteinn

llvm/lib/Analysis/ValueTracking.cpp

@@ -2932,6 +2932,10 @@ bool isKnownNonZero(const Value *V, const APInt &DemandedElts, unsigned Depth,
       case Intrinsic::ssub_sat:
         return isNonZeroSub(DemandedElts, Depth, Q, BitWidth,
                             II->getArgOperand(0), II->getArgOperand(1));
+      case Intrinsic::sadd_sat:
+        return isNonZeroAdd(DemandedElts, Depth, Q, BitWidth,
+                            II->getArgOperand(0), II->getArgOperand(1),
+                            /*NSW*/ true);
       case Intrinsic::uadd_sat:
         if (isKnownNonZero(II->getArgOperand(0), DemandedElts, Depth, Q) ||
             isKnownNonZero(II->getArgOperand(1), DemandedElts, Depth, Q))

llvm/test/Analysis/ValueTracking/known-non-zero.ll

@@ -924,10 +924,7 @@ define i1 @sadd_sat_nonzero(i8 %xx, i8 %yy, i8 %ind) {
 ; CHECK-NEXT:    [[Y_POS:%.*]] = icmp sge i8 [[YY:%.*]], 0
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[X_STRICT_POS]])
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[Y_POS]])
-; CHECK-NEXT:    [[X:%.*]] = call i8 @llvm.sadd.sat.i8(i8 [[XX]], i8 [[YY]])
-; CHECK-NEXT:    [[Z:%.*]] = or i8 [[X]], [[IND:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[Z]], 0
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 false
 ;
   %x_strict_pos = icmp sgt i8 %xx, 0
   %y_pos = icmp sge i8 %yy, 0