VM: Small generated code size improvements on x64.

runtime/vm/flow_graph_compiler_x64.cc

Index: runtime/vm/flow_graph_compiler_x64.cc
===================================================================
--- runtime/vm/flow_graph_compiler_x64.cc	(revision 42687)
+++ runtime/vm/flow_graph_compiler_x64.cc	(working copy)
@@ -1249,7 +1249,7 @@
                    &target_label,
                    RawPcDescriptors::kUnoptStaticCall,
                    locs);
-  __ Drop(argument_count);
+  __ Drop(argument_count, RCX);
 #if defined(DEBUG)
   __ movq(RBX, Immediate(kInvalidObjectPointer));
   __ movq(R10, Immediate(kInvalidObjectPointer));
@@ -1311,7 +1311,7 @@
                    target_label,
                    RawPcDescriptors::kIcCall,
                    locs);
-  __ Drop(argument_count);
+  __ Drop(argument_count, RCX);
 }
 
 
@@ -1328,7 +1328,7 @@
                    target_label,
                    RawPcDescriptors::kIcCall,
                    locs);
-  __ Drop(argument_count);
+  __ Drop(argument_count, RCX);
 #if defined(DEBUG)
   __ movq(R10, Immediate(kInvalidObjectPointer));
 #endif
@@ -1392,7 +1392,7 @@
       Isolate::kNoDeoptId, token_pos);
   RecordSafepoint(locs);
   AddDeoptIndexAtCall(Isolate::ToDeoptAfter(deopt_id), token_pos);
-  __ Drop(argument_count);
+  __ Drop(argument_count, RCX);
 }
 
 
@@ -1413,7 +1413,7 @@
                    RawPcDescriptors::kOptStaticCall,
                    locs);
   AddStaticCallTarget(function);
-  __ Drop(argument_count);
+  __ Drop(argument_count, RCX);
 }
 
 
@@ -1565,7 +1565,7 @@
                      locs);
     const Function& function = *sorted[i].target;
     AddStaticCallTarget(function);
-    __ Drop(argument_count);
+    __ Drop(argument_count, RCX);
     if (!is_last_check) {
       __ jmp(&match_found);
     }