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/mir_graph.cc b/compiler/dex/mir_graph.cc
index a2676c8..63a5570 100644
--- a/compiler/dex/mir_graph.cc
+++ b/compiler/dex/mir_graph.cc
@@ -586,7 +586,7 @@
   if (current_method_ == 0) {
     DCHECK(entry_block_ == NULL);
     DCHECK(exit_block_ == NULL);
-    DCHECK_EQ(num_blocks_, 0);
+    DCHECK_EQ(num_blocks_, 0U);
     // Use id 0 to represent a null block.
     BasicBlock* null_block = NewMemBB(kNullBlock, num_blocks_++);
     DCHECK_EQ(null_block->id, NullBasicBlockId);