Fix a performance bug in the register contraints for polymorphic loads.

src/ia32/lithium-ia32.cc

Index: src/ia32/lithium-ia32.cc
===================================================================
--- src/ia32/lithium-ia32.cc	(revision 10817)
+++ src/ia32/lithium-ia32.cc	(working copy)
@@ -1935,13 +1935,14 @@
 LInstruction* LChunkBuilder::DoLoadNamedFieldPolymorphic(
     HLoadNamedFieldPolymorphic* instr) {
   ASSERT(instr->representation().IsTagged());
-  LOperand* context = UseFixed(instr->context(), esi);
   if (instr->need_generic()) {
+    LOperand* context = UseFixed(instr->context(), esi);
     LOperand* obj = UseFixed(instr->object(), eax);
     LLoadNamedFieldPolymorphic* result =
         new(zone()) LLoadNamedFieldPolymorphic(context, obj);
     return MarkAsCall(DefineFixed(result, eax), instr);
   } else {
+    LOperand* context = UseAny(instr->context());  // Not actually used.
     LOperand* obj = UseRegisterAtStart(instr->object());
     LLoadNamedFieldPolymorphic* result =
         new(zone()) LLoadNamedFieldPolymorphic(context, obj);