Use r15 instead of r13 in generic return dec-refs

This avoids a bad interaction with LinearScan, which saves the return
value in r13 and r14 around GenericRetDecRefs.  Effectively, this diff
ends up saving a push, a pop, and some stack adjustment in
GenericRetDecRefs.

hphp/runtime/vm/translator/hopt/codegen.cpp

@@ -2706,13 +2706,13 @@ void CodeGenerator::cgGenericRetDecRefs(IRInstruction* inst) {
 
   if (numLocals == 0) return;
 
-  // The helpers called below use a special ABI, in which r13 is not saved.
-  // So save r13 on the stack if it's live.
-  bool saveR13 = m_state.liveRegs[inst].contains(r13);
+  // The helpers called below use a special ABI, in which r15 is not saved.
+  // So save r15 on the stack if it's live.
+  bool saveR15 = m_state.liveRegs[inst].contains(r15);
 
   int stackAdjust = 8;
-  if (saveR13)  {
-    a.push(r13);
+  if (saveR15)  {
+    a.push(r15);
     stackAdjust = 16;
   }
 
@@ -2725,9 +2725,9 @@ void CodeGenerator::cgGenericRetDecRefs(IRInstruction* inst) {
   a.call(target);
   recordSyncPoint(a);
 
-  if (saveR13) {
+  if (saveR15) {
     a.addq(8, rsp);
-    a.pop(r13);
+    a.pop(r15);
   }
 }
 

hphp/runtime/vm/translator/translator-x64.cpp

@@ -11000,12 +11000,12 @@ void TranslatorX64::emitFreeLocalsHelpers() {
   Label loopHead;
 
   /*
-   * Note: the IR currently requires that we preserve r14/r15 across
+   * Note: the IR currently requires that we preserve r13/r14 across
    * calls to these free locals helpers.
    */
   static_assert(rVmSp == rbx, "");
   auto const rIter     = rbx;
-  auto const rFinished = r13;
+  auto const rFinished = r15;
   auto const rType     = esi;
   auto const rData     = rdi;