[MERGE #5402 @Penguinwizzard] Fix typo in cleanup for poisoning code

Merge pull request #5402 from Penguinwizzard:backport_length_offset_fix

Backporting this fix to release/1.8 due to special request.

lib/Backend/Lower.cpp

@@ -16266,7 +16266,7 @@ Lowerer::GenerateFastElemIIntIndexCommon(
             if (baseValueType.IsLikelyTypedArray())
             {
                 int lengthOffset;
-                lengthOffset = GetArrayOffsetOfHeadSegment(baseValueType);
+                lengthOffset = GetArrayOffsetOfLength(baseValueType);
                 headSegmentLengthOpnd = IR::IndirOpnd::New(arrayOpnd, lengthOffset, TyUint32, m_func);
                 autoReuseHeadSegmentLengthOpnd.Initialize(headSegmentLengthOpnd, m_func);
             }