Simplify and fix code aging.

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index c430465070da9ac967a53dc441c251fef2a002fc..9217a9407fc942e2ea077bec5cee802b8737299b 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -133,8 +133,6 @@ bool LCodeGen::GeneratePrologue() {
   // object). rcx is zero for method calls and non-zero for function
   // calls.
   if (!info_->is_classic_mode() || info_->is_native()) {
-    Label begin;
-    __ bind(&begin);
     Label ok;
     __ testq(rcx, rcx);
     __ j(zero, &ok, Label::kNear);
@@ -143,10 +141,9 @@ bool LCodeGen::GeneratePrologue() {
     __ LoadRoot(kScratchRegister, Heap::kUndefinedValueRootIndex);
     __ movq(Operand(rsp, receiver_offset), kScratchRegister);
     __ bind(&ok);
-    ASSERT(!FLAG_age_code ||
-           (kSizeOfOptimizedStrictModePrologue == ok.pos() - begin.pos()));
   }
 
+  info()->set_prologue_offset(masm_->pc_offset());
   __ push(rbp);  // Caller's frame pointer.
   __ movq(rbp, rsp);
   __ push(rsi);  // Callee's context.