Reduce time and memory usage of GVN.

Filter out dead sregs in GVN. Reclaim memory after each LVN
in the GVN modification phase.

Bug: 16398693
Change-Id: I8c88c3009663754e1b66c0ef3f62c3b93276e385
diff --git a/compiler/dex/frontend.cc b/compiler/dex/frontend.cc
index 51446f6..11dd182 100644
--- a/compiler/dex/frontend.cc
+++ b/compiler/dex/frontend.cc
@@ -747,7 +747,7 @@
 
   /* Free Arenas from the cu.arena_stack for reuse by the cu.arena in the codegen. */
   if (cu.enable_debug & (1 << kDebugShowMemoryUsage)) {
-    if (cu.arena_stack.PeakBytesAllocated() > 256 * 1024) {
+    if (cu.arena_stack.PeakBytesAllocated() > 1 * 1024 * 1024) {
       MemStats stack_stats(cu.arena_stack.GetPeakStats());
       LOG(INFO) << method_name << " " << Dumpable<MemStats>(stack_stats);
     }