Merging r168291: into the 3.2 release branch.

Fix PR14060, an infinite loop in reassociate.  The problem was that one of the
operands of the expression being written was wrongly thought to be reusable as
an inner node of the expression resulting in it turning up as both an inner node
*and* a leaf, creating a cycle in the def-use graph.  This would have caused the
verifier to blow up if things had gotten that far, however it managed to provoke
an infinite loop first.

llvm-svn: 168489