[LVI] Check ConstantFoldCompareInstOperands() failure (NFCI)

I don't believe this can happen right now (because we're only
working on icmps and as such can't hit the current fcmp null
paths), but this will be possible in the future when icmp
constant expressions are removed.

llvm/lib/Analysis/LazyValueInfo.cpp

@@ -1749,7 +1749,7 @@ getPredicateResult(unsigned Pred, Constant *C, const ValueLatticeElement &Val,
   Constant *Res = nullptr;
   if (Val.isConstant()) {
     Res = ConstantFoldCompareInstOperands(Pred, Val.getConstant(), C, DL, TLI);
-    if (ConstantInt *ResCI = dyn_cast<ConstantInt>(Res))
+    if (ConstantInt *ResCI = dyn_cast_or_null<ConstantInt>(Res))
       return ResCI->isZero() ? LazyValueInfo::False : LazyValueInfo::True;
     return LazyValueInfo::Unknown;
   }
@@ -1791,14 +1791,14 @@ getPredicateResult(unsigned Pred, Constant *C, const ValueLatticeElement &Val,
       Res = ConstantFoldCompareInstOperands(ICmpInst::ICMP_NE,
                                             Val.getNotConstant(), C, DL,
                                             TLI);
-      if (Res->isNullValue())
+      if (Res && Res->isNullValue())
         return LazyValueInfo::False;
     } else if (Pred == ICmpInst::ICMP_NE) {
       // !C1 != C -> true iff C1 == C.
       Res = ConstantFoldCompareInstOperands(ICmpInst::ICMP_NE,
                                             Val.getNotConstant(), C, DL,
                                             TLI);
-      if (Res->isNullValue())
+      if (Res && Res->isNullValue())
         return LazyValueInfo::True;
     }
     return LazyValueInfo::Unknown;