Revert GCC compilation fix on 3.1 branch that contained an invalid

src/x64/codegen-x64.cc

Index: src/x64/codegen-x64.cc
===================================================================
--- src/x64/codegen-x64.cc	(revision 7609)
+++ src/x64/codegen-x64.cc	(working copy)
@@ -8195,9 +8195,17 @@
     result = allocator()->Allocate();
     ASSERT(result.is_valid());
 
-    // r12 is now a reserved register, so it cannot be the receiver.
-    // If it was, the distance to the fixup location would not be constant.
-    ASSERT(!receiver.reg().is(r12));
+    // Cannot use r12 for receiver, because that changes
+    // the distance between a call and a fixup location,
+    // due to a special encoding of r12 as r/m in a ModR/M byte.
+    if (receiver.reg().is(r12)) {
+      frame()->Spill(receiver.reg());  // It will be overwritten with result.
+      // Swap receiver and value.
+      __ movq(result.reg(), receiver.reg());
+      Result temp = receiver;
+      receiver = result;
+      result = temp;
+    }
 
     DeferredReferenceGetNamedValue* deferred =
         new DeferredReferenceGetNamedValue(result.reg(), receiver.reg(), name);