[SelectionDAG] Restore calls to has_value (NFC)

[kazu]

Summary

This patch restores calls to has_value to make it clear that we are
checking the presence of an optional value, not the underlying value.

This patch partially reverts d08f34b592ff06ccb1f36da88ec09aa926427a4d.

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

bool DAGCombiner::mayAlias(SDNode *Op0, SDNode *Op1) const {
   // multiples of the size of the data.
   int64_t SrcValOffset0 = MUC0.MMO->getOffset();
   int64_t SrcValOffset1 = MUC1.MMO->getOffset();
   Align OrigAlignment0 = MUC0.MMO->getBaseAlign();
   Align OrigAlignment1 = MUC1.MMO->getBaseAlign();
   auto &Size0 = MUC0.NumBytes;
   auto &Size1 = MUC1.NumBytes;
   if (OrigAlignment0 == OrigAlignment1 && SrcValOffset0 != SrcValOffset1 &&
-      Size0 && Size1 && *Size0 == *Size1 && OrigAlignment0 > *Size0 &&
-      SrcValOffset0 % *Size0 == 0 && SrcValOffset1 % *Size1 == 0) {
+      Size0.has_value() && Size1.has_value() && *Size0 == *Size1 &&
+      OrigAlignment0 > *Size0 && SrcValOffset0 % *Size0 == 0 &&
+      SrcValOffset1 % *Size1 == 0) {
     int64_t OffAlign0 = SrcValOffset0 % OrigAlignment0.value();
     int64_t OffAlign1 = SrcValOffset1 % OrigAlignment1.value();
 
     // There is no overlap between these relatively aligned accesses of
     // similar size. Return no alias.
     if ((OffAlign0 + *Size0) <= OffAlign1 || (OffAlign1 + *Size1) <= OffAlign0)
       return false;
   }