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

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
===================================================================
--- runtime/vm/flow_graph_compiler_x64.cc	(revision 44981)
+++ runtime/vm/flow_graph_compiler_x64.cc	(working copy)
@@ -1232,10 +1232,6 @@
                    RawPcDescriptors::kUnoptStaticCall,
                    locs);
   __ Drop(argument_count, RCX);
-#if defined(DEBUG)
-  __ movq(RBX, Immediate(kInvalidObjectPointer));
-  __ movq(R10, Immediate(kInvalidObjectPointer));
-#endif
 }
 
 
@@ -1306,9 +1302,6 @@
                    RawPcDescriptors::kIcCall,
                    locs);
   __ Drop(argument_count, RCX);
-#if defined(DEBUG)
-  __ movq(R10, Immediate(kInvalidObjectPointer));
-#endif
 }
 
 
@@ -1450,13 +1443,6 @@
                            Isolate::kNoDeoptId,
                            token_pos);
     }
-#if defined(DEBUG)
-    // Do this *after* adding the pc descriptor!
-    if (!is_optimizing()) {
-      __ movq(R10, Immediate(kInvalidObjectPointer));
-      __ movq(RBX, Immediate(kInvalidObjectPointer));
-    }
-#endif
     // Stub returns result in flags (result of a cmpq, we need ZF computed).
     __ popq(right);
     __ popq(left);