Analysis
========
ALTER TABLE operations having composite indexes can cause
rebuild of such indexes if:
a) First index component is non-prefixed index and
b) Subsequent index components have at least one prefixed
   index.

The code which checked if the index had changed from
non-prefixed index to prefixed index, checked only the
first key part and did not scan all key parts to determine
whether the original version of the index was prefixed
or not. Since the composite index's first key part was
not prefixed and the new index definition had the
prefix flag set, it assumed there is a change in
the index definition triggering a rebuild of the index.

Fix
===
Scan all the key parts in order to determine whether
the key is converted from a non-prefixed key to a
prefixed key or vice-versa.