ART: More warnings

Enable -Wno-conversion-null, -Wredundant-decls and -Wshadow in general,
and -Wunused-but-set-parameter for GCC builds.

Change-Id: I81bbdd762213444673c65d85edae594a523836e5
diff --git a/compiler/optimizing/gvn.cc b/compiler/optimizing/gvn.cc
index 027b3d4..25168b5 100644
--- a/compiler/optimizing/gvn.cc
+++ b/compiler/optimizing/gvn.cc
@@ -54,8 +54,9 @@
 
     SideEffects effects = SideEffects::None();
     // Update `effects` with the side effects of all instructions in this block.
-    for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
-      HInstruction* instruction = it.Current();
+    for (HInstructionIterator inst_it(block->GetInstructions()); !inst_it.Done();
+         inst_it.Advance()) {
+      HInstruction* instruction = inst_it.Current();
       effects = effects.Union(instruction->GetSideEffects());
       if (effects.HasAllSideEffects()) {
         break;