Cleanup: use JumpIf[Not]Smi() whenever we can

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
diff --git a/src/x64/full-codegen-x64.cc b/src/x64/full-codegen-x64.cc
index 6c2aa3e05c01a50e3d8d67baebe279c3c859032a..c347699f8e7fe8c592f760090058ddf7be5c42e9 100644
--- a/src/x64/full-codegen-x64.cc
+++ b/src/x64/full-codegen-x64.cc
@@ -3710,8 +3710,7 @@ void FullCodeGenerator::VisitUnaryOperation(UnaryOperation* expr) {
       Comment cmt(masm_, "[ UnaryOperation (ADD)");
       VisitForAccumulatorValue(expr->expression());
       Label no_conversion;
-      Condition is_smi = masm_->CheckSmi(result_register());
-      __ j(is_smi, &no_conversion);
+      __ JumpIfSmi(result_register(), &no_conversion);
       ToNumberStub convert_stub;
       __ CallStub(&convert_stub);
       __ bind(&no_conversion);
@@ -3814,9 +3813,7 @@ void FullCodeGenerator::VisitCountOperation(CountOperation* expr) {
 
   // Call ToNumber only if operand is not a smi.
   Label no_conversion;
-  Condition is_smi;
-  is_smi = masm_->CheckSmi(rax);
-  __ j(is_smi, &no_conversion, Label::kNear);
+  __ JumpIfSmi(rax, &no_conversion, Label::kNear);
   ToNumberStub convert_stub;
   __ CallStub(&convert_stub);
   __ bind(&no_conversion);
@@ -4184,8 +4181,7 @@ void FullCodeGenerator::VisitCompareToNull(CompareToNull* expr) {
     __ j(equal, if_true);
     __ CompareRoot(rax, Heap::kUndefinedValueRootIndex);
     __ j(equal, if_true);
-    Condition is_smi = masm_->CheckSmi(rax);
-    __ j(is_smi, if_false);
+    __ JumpIfSmi(rax, if_false);
     // It can be an undetectable object.
     __ movq(rdx, FieldOperand(rax, HeapObject::kMapOffset));
     __ testb(FieldOperand(rdx, Map::kBitFieldOffset),