Reset _defMergedNodes in global VP on every basic block

_defMergedNodes is supposed to keep track of commonned nodes.
So it's safe to reset it on every basic block. Also add an assert
that there are no extended blocks during global VP.

compiler/optimizer/OMRValuePropagation.cpp

@@ -4081,7 +4081,6 @@ void TR::GlobalValuePropagation::processStructure(TR_StructureSubGraphNode *node
    TR_RegionStructure *region = node->getStructure()->asRegion();
    if (region)
       {
-      _defMergedNodes->empty();
       if (region->isAcyclic())
          {
          processAcyclicRegion(node, lastTimeThrough, insideLoop);
@@ -4097,6 +4096,10 @@ void TR::GlobalValuePropagation::processStructure(TR_StructureSubGraphNode *node
       }
    else
       {
+      // Only commoned nodes within a block need to be set in _defMergedNodes.
+      // So it should be reset here
+      TR_ASSERT_FATAL(!node->getStructure()->asBlock()->getBlock()->isExtensionOfPreviousBlock(), "This optimization does not run on extended blocks");
+      _defMergedNodes->empty();
       processBlock(node, lastTimeThrough, insideLoop);
       }
    }