Prevent MVC reduction in astoreEvaluator if symref is unresolved

The `astoreEvaluator`, more precisely the `astoreHelper` has a path in
which it is trying to identify the following tree pattern:

```
astore <x>
  aload <y>
```

and tries to generate an `MVC` instruction to perform the store. The
problem with this is that it seems to generate a memory reference from
the `node` only for the purposes of testing whether it has an index
register (which it by the way cannot know until the memory reference is
used within an instruction). The act of generating a memory reference
is not side-effect free as the symref could have been unresolved and as
such various metadata can be generated.

The `astoreHelper` then goes ahead and creates another memory reference
from the node and uses that for the `MVC` instruction. This means the
metadata for handling unresolved symrefs could have been generated
twice.

Similarly to `directMemoryStoreHelper` we simply prevent this `MVC`
reduction if the symref is unresolved.

Signed-off-by: Filip Jeremic <fjeremic@ca.ibm.com>

compiler/z/codegen/OMRTreeEvaluator.cpp

@@ -5693,7 +5693,7 @@ astoreHelper(TR::Node * node, TR::CodeGenerator * cg)
             }
          }
       // aload is the child, then don't evaluate the child, generate MVC to move directly among memory
-      else if(!node->getOpCode().isIndirect() &&
+      else if(!node->getOpCode().isIndirect() && !node->getSymbolReference()->isUnresolved() &&
               valueChild->getOpCodeValue() == TR::aload &&
               valueChild->getReferenceCount() == 1 &&
               valueChild->getRegister() == NULL &&