Fix two x64 code generation bugs introduced by const context slot code.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
===================================================================
--- src/x64/lithium-codegen-x64.cc	(revision 10244)
+++ src/x64/lithium-codegen-x64.cc	(working copy)
@@ -2074,7 +2074,7 @@
     } else {
       Label is_not_hole;
       __ j(not_equal, &is_not_hole, Label::kNear);
-      __ movq(result, factory()->undefined_value(), RelocInfo::NONE);
+      __ LoadRoot(result, Heap::kUndefinedValueRootIndex);
       __ bind(&is_not_hole);
     }
   }
@@ -2093,7 +2093,7 @@
     if (instr->hydrogen()->DeoptimizesOnHole()) {
       DeoptimizeIf(equal, instr->environment());
     } else {
-      __ j(not_equal, &skip_assignment, Label::kNear);
+      __ j(not_equal, &skip_assignment);
     }
   }
   __ movq(target, value);