X64: Do not use an AllocateWithoutSpill register if it is invalid.

src/x64/codegen-x64.cc

Index: src/x64/codegen-x64.cc
===================================================================
--- src/x64/codegen-x64.cc	(revision 2675)
+++ src/x64/codegen-x64.cc	(working copy)
@@ -3084,26 +3084,36 @@
                                                   is_increment);
     }
 
+    // If we have a free register, combine the smi and overflow checks.
     Result tmp = allocator_->AllocateWithoutSpilling();
     ASSERT(kSmiTagMask == 1 && kSmiTag == 0);
-    __ movl(tmp.reg(), Immediate(kSmiTagMask));
-    // Smi test.
+    if (tmp.is_valid()) {
+      __ movl(tmp.reg(), Immediate(kSmiTagMask));
+    }
+
+    // Try incrementing or decrementing the smi.
     __ movq(kScratchRegister, new_value.reg());
     if (is_increment) {
       __ addl(kScratchRegister, Immediate(Smi::FromInt(1)));
     } else {
       __ subl(kScratchRegister, Immediate(Smi::FromInt(1)));
     }
-    // deferred->Branch(overflow);
-    __ cmovl(overflow, kScratchRegister, tmp.reg());
-    __ testl(kScratchRegister, tmp.reg());
-    tmp.Unuse();
-    deferred->Branch(not_zero);
+
+    // Go to the deferred case if the result overflows or is non-smi.
+    if (tmp.is_valid()){
+      __ cmovl(overflow, kScratchRegister, tmp.reg());
+      __ testl(kScratchRegister, tmp.reg());
+      tmp.Unuse();
+      deferred->Branch(not_zero);
+    } else {
+      deferred->Branch(overflow);
+      __ testl(kScratchRegister, Immediate(kSmiTagMask));
+      deferred->Branch(not_zero);
+    }
+
     __ movq(new_value.reg(), kScratchRegister);
-
     deferred->BindExit();
 
-
     // Postfix: store the old value in the allocated slot under the
     // reference.
     if (is_postfix) frame_->SetElementAt(target.size(), &old_value);