X87: Reland "Merge cellspace into old pointer space".

src/x87/assembler-x87-inl.h

Index: src/x87/assembler-x87-inl.h
diff --git a/src/x87/assembler-x87-inl.h b/src/x87/assembler-x87-inl.h
index 800ead2803c2a430f488133e2fd088d2a8b111c2..4f5768e684df8b3d54b83a00d6ba98e808038d7d 100644
--- a/src/x87/assembler-x87-inl.h
+++ b/src/x87/assembler-x87-inl.h
@@ -205,6 +205,7 @@ Cell* RelocInfo::target_cell() {
 void RelocInfo::set_target_cell(Cell* cell,
                                 WriteBarrierMode write_barrier_mode,
                                 ICacheFlushMode icache_flush_mode) {
+  DCHECK(cell->IsCell());
   DCHECK(rmode_ == RelocInfo::CELL);
   Address address = cell->address() + Cell::kValueOffset;
   Memory::Address_at(pc_) = address;