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/dex/ssa_transformation.cc b/compiler/dex/ssa_transformation.cc
index 412f85d..d3d76ba 100644
--- a/compiler/dex/ssa_transformation.cc
+++ b/compiler/dex/ssa_transformation.cc
@@ -539,8 +539,7 @@
     for (BasicBlockId pred_id : bb->predecessors) {
       BasicBlock* pred_bb = GetBasicBlock(pred_id);
       DCHECK(pred_bb != nullptr);
-      int ssa_reg = pred_bb->data_flow_info->vreg_to_ssa_map_exit[v_reg];
-      uses[idx] = ssa_reg;
+      uses[idx] = pred_bb->data_flow_info->vreg_to_ssa_map_exit[v_reg];
       incoming[idx] = pred_id;
       idx++;
     }