Remove zapping of registers that has not turned up any GC bugs in a long time.

runtime/vm/flow_graph_compiler_arm64.cc

Index: runtime/vm/flow_graph_compiler_arm64.cc
===================================================================
--- runtime/vm/flow_graph_compiler_arm64.cc	(revision 44981)
+++ runtime/vm/flow_graph_compiler_arm64.cc	(working copy)
@@ -1253,9 +1253,6 @@
                    RawPcDescriptors::kIcCall,
                    locs);
   __ Drop(argument_count);
-#if defined(DEBUG)
-  __ LoadImmediate(R4, kInvalidObjectPointer, kNoPP);
-#endif
 }
 
 
@@ -1337,10 +1334,6 @@
                    &target_label,
                    RawPcDescriptors::kUnoptStaticCall,
                    locs);
-#if defined(DEBUG)
-  __ LoadImmediate(R4, kInvalidObjectPointer, kNoPP);
-  __ LoadImmediate(R5, kInvalidObjectPointer, kNoPP);
-#endif
   __ Drop(argument_count);
 }
 
@@ -1418,13 +1411,6 @@
                            Isolate::kNoDeoptId,
                            token_pos);
     }
-#if defined(DEBUG)
-    if (!is_optimizing()) {
-      // Do this *after* adding the pc descriptor!
-      __ LoadImmediate(R4, kInvalidObjectPointer, kNoPP);
-      __ LoadImmediate(R5, kInvalidObjectPointer, kNoPP);
-    }
-#endif
     // Stub returns result in flags (result of a cmp, we need Z computed).
     __ Pop(right);
     __ Pop(left);