6965815: OptimizeStringConcat: assert(!q->is_MergeMem()) failed with specjbb2000

Reviewed-by: kvn

src/share/vm/opto/graphKit.cpp

@@ -1739,6 +1739,7 @@ void GraphKit::replace_call(CallNode* call, Node* result) {
   C->gvn_replace_by(callprojs.fallthrough_catchproj, final_state->in(TypeFunc::Control));
   C->gvn_replace_by(callprojs.fallthrough_memproj,   final_state->in(TypeFunc::Memory));
   C->gvn_replace_by(callprojs.fallthrough_ioproj,    final_state->in(TypeFunc::I_O));
+  Node* final_mem = final_state->in(TypeFunc::Memory);
 
   // Replace the result with the new result if it exists and is used
   if (callprojs.resproj != NULL && result != NULL) {
@@ -1776,6 +1777,21 @@ void GraphKit::replace_call(CallNode* call, Node* result) {
   // Disconnect the call from the graph
   call->disconnect_inputs(NULL);
   C->gvn_replace_by(call, C->top());
+
+  // Clean up any MergeMems that feed other MergeMems since the
+  // optimizer doesn't like that.
+  if (final_mem->is_MergeMem()) {
+    Node_List wl;
+    for (SimpleDUIterator i(final_mem); i.has_next(); i.next()) {
+      Node* m = i.get();
+      if (m->is_MergeMem() && !wl.contains(m)) {
+        wl.push(m);
+      }
+    }
+    while (wl.size()  > 0) {
+      _gvn.transform(wl.pop());
+    }
+  }
 }