Fix Smi-based MathMinMax on x64, and reenable smi mode.

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index d6f05c0a2df80bf90531fda2ed89aae021f9c442..a6f9801c831638a070d0b47ab56fde41666e26b0 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -1562,9 +1562,9 @@ LInstruction* LChunkBuilder::DoAdd(HAdd* instr) {
 LInstruction* LChunkBuilder::DoMathMinMax(HMathMinMax* instr) {
   LOperand* left = NULL;
   LOperand* right = NULL;
-  if (instr->representation().IsInteger32()) {
-    ASSERT(instr->left()->representation().IsInteger32());
-    ASSERT(instr->right()->representation().IsInteger32());
+  if (instr->representation().IsSmiOrInteger32()) {
+    ASSERT(instr->left()->representation().Equals(instr->representation()));
+    ASSERT(instr->right()->representation().Equals(instr->representation()));
     left = UseRegisterAtStart(instr->BetterLeftOperand());
     right = UseOrConstantAtStart(instr->BetterRightOperand());
   } else {