[mlir] Don't return nullptrs from scf::IfOp::getSuccessorRegions

[tpopp]

Summary

Previously this might happen if there was no elseRegion and the method
was asked for all successor regions.

mlir/lib/Dialect/SCF/SCF.cpp

void IfOp::getSuccessorRegions(Optional<unsigned> index,
 
   // Otherwise, the successor is dependent on the condition.
   bool condition;
   if (auto condAttr = operands.front().dyn_cast_or_null<IntegerAttr>()) {
     condition = condAttr.getValue().isOneValue();
   } else {
     // If the condition isn't constant, both regions may be executed.
     regions.push_back(RegionSuccessor(&thenRegion()));
+    // If the else region does not exist, it is not a viable successor.
+    if (elseRegion)
       regions.push_back(RegionSuccessor(elseRegion));
     return;
   }
 
   // Add the successor regions using the condition.
   regions.push_back(RegionSuccessor(condition ? &thenRegion() : elseRegion));
 }
 
 namespace {