Fix MPAOL inconsist issue between freeBytes and freeList

inconsist issue caused by missing to update previous free entry
during elimilating free entry due to card alignment, it would
generate gap between previous and current free entry for
recycleHeapChunk, then cause losing free entries. it could trigger
the assertions of gc allocation for balanced gc case.

 - updated previous free entry for the case
 - additional assertion check in recycleHeapChunk for verifying
 the case -- for debugging only.

Signed-off-by: Lin Hu <linhu@ca.ibm.com>

gc/base/MemoryPoolAddressOrderedList.cpp

@@ -456,6 +456,8 @@ MM_MemoryPoolAddressOrderedList::internalAllocate(MM_EnvironmentBase *env, uintp
 			currentFreeEntry = doFreeEntryAlignmentUpTo(env, currentFreeEntry);
 			if (NULL == currentFreeEntry) {
 				currentFreeEntry = (FREE_ENTRY_END == _firstUnalignedFreeEntry) ? NULL : _firstUnalignedFreeEntry;
+				previousFreeEntry = (FREE_ENTRY_END == _prevFirstUnalignedFreeEntry) ? NULL : _prevFirstUnalignedFreeEntry;
+				walkCount += 1;
 				continue;
 			}
 		}
@@ -1352,6 +1354,16 @@ MM_MemoryPoolAddressOrderedList::recycleHeapChunk(
 	bool const compressed = compressObjectReferences();
 	Assert_MM_true(addrBase <= addrTop);
 	Assert_MM_true((NULL == nextFreeEntry) || (addrTop <= nextFreeEntry));
+#if defined(DEBUG)
+	/* confirming there is no gap between previous free entry and addrBase */
+	MM_HeapLinkedFreeHeader *current = NULL;
+	if (previousFreeEntry) {
+		current = previousFreeEntry->getNext(compressed);
+	} else if (_heapFreeList) {
+		current = _heapFreeList;
+	}
+	Assert_MM_true((current == NULL) || (((uintptr_t)current + current->getSize()) >= (uintptr_t)addrBase));
+#endif
 	if (internalRecycleHeapChunk(addrBase, addrTop, nextFreeEntry)) {
 		if (previousFreeEntry) {
 			Assert_MM_true(previousFreeEntry < addrBase);