Add TransformExtendCombine which is used to flatten extend and assign operations together where possible.

Note that normal algebra construction does this automatically but in some cases it is useful to have this as a separate transform that may be applied

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1516517 13f79535-47bb-0310-9956-ffa450edef68