ART: Hide unreachable basic blocks in the compiler

Unreachable blocks are not handled uniformly in the optimization
passes. Uniformly hide them to avoid initialization errors.

Bug: 15573463
Change-Id: Ia9e89fa357d5672a6cd8389f28a06ff618fe60ee
diff --git a/compiler/dex/frontend.cc b/compiler/dex/frontend.cc
index d453797..ca15964 100644
--- a/compiler/dex/frontend.cc
+++ b/compiler/dex/frontend.cc
@@ -756,7 +756,7 @@
       support_list_size = arraysize(x86_64_support_list);
     }
 
-    for (int idx = 0; idx < cu.mir_graph->GetNumBlocks(); idx++) {
+    for (unsigned int idx = 0; idx < cu.mir_graph->GetNumBlocks(); idx++) {
       BasicBlock* bb = cu.mir_graph->GetBasicBlock(idx);
       if (bb == NULL) continue;
       if (bb->block_type == kDead) continue;