Avoid allocating unnecessary result locations for some instructions.

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
===================================================================
--- runtime/vm/intermediate_language_x64.cc	(revision 12100)
+++ runtime/vm/intermediate_language_x64.cc	(working copy)
@@ -1055,7 +1055,9 @@
   LocationSummary* locs = new LocationSummary(1, 1, LocationSummary::kNoCall);
   locs->set_in(0, Location::RequiresRegister());
   locs->set_temp(0, Location::RequiresRegister());
-  locs->set_out(Location::SameAsFirstInput());
+  locs->set_out(is_used()
+                ? Location::SameAsFirstInput()
+                : Location::NoLocation());
   return locs;
 }
 
@@ -1063,7 +1065,7 @@
 void StoreStaticFieldInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   Register value = locs()->in(0).reg();
   Register temp = locs()->temp(0).reg();
-  ASSERT(locs()->out().reg() == value);
+  ASSERT(!is_used() || (locs()->out().reg() == value));
 
   __ LoadObject(temp, field());
   if (this->value()->NeedsStoreBuffer()) {