Fix invalid out-of-bounds store in MacroAssembler::Allocate.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index 9c9b1620e586ce5872bbc46de2eaf515266716a0..74e3fcc33a1701d4a1836090188de51776bc22f5 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -3859,7 +3859,7 @@ void MacroAssembler::Allocate(int object_size,
   LoadAllocationTopHelper(result, scratch, flags);
 
   // Align the next allocation. Storing the filler map without checking top is
-  // always safe because the limit of the heap is always aligned.
+  // safe in new-space because the limit of the heap is aligned there.
   if (((flags & DOUBLE_ALIGNMENT) != 0) && FLAG_debug_code) {
     testq(result, Immediate(kDoubleAlignmentMask));
     Check(zero, kAllocationIsNotDoubleAligned);
@@ -3938,7 +3938,7 @@ void MacroAssembler::Allocate(Register object_size,
   LoadAllocationTopHelper(result, scratch, flags);
 
   // Align the next allocation. Storing the filler map without checking top is
-  // always safe because the limit of the heap is always aligned.
+  // safe in new-space because the limit of the heap is aligned there.
   if (((flags & DOUBLE_ALIGNMENT) != 0) && FLAG_debug_code) {
     testq(result, Immediate(kDoubleAlignmentMask));
     Check(zero, kAllocationIsNotDoubleAligned);