DBC: Handle constants correctly in the optimizing backend

runtime/vm/intermediate_language_dbc.cc

Index: runtime/vm/intermediate_language_dbc.cc
diff --git a/runtime/vm/intermediate_language_dbc.cc b/runtime/vm/intermediate_language_dbc.cc
index 859f7d432f3faa8106d55636a27ca97ab0e79fbd..ec13a8339869a875de4b355bfe2cd7fdfb11c64e 100644
--- a/runtime/vm/intermediate_language_dbc.cc
+++ b/runtime/vm/intermediate_language_dbc.cc
@@ -339,7 +339,9 @@ EMIT_NATIVE_CODE(LoadClassId, 1, Location::RequiresRegister()) {
 
 EMIT_NATIVE_CODE(Constant, 0, Location::RequiresRegister()) {
   if (compiler->is_optimizing()) {
-    __ LoadConstant(locs()->out(0).reg(), value());
+    if (locs()->out(0).IsRegister()) {
+      __ LoadConstant(locs()->out(0).reg(), value());
+    }
   } else {
     __ PushConstant(value());
   }