Add a sceleton for a polyhedral dead code elimination.

Such a dead code elimination can remove redundant stores to arrays. It can also
eliminate calculations where the results are stored to memory but where they are
overwritten before ever being read. It may also fix bugs like:
http://llvm.org/bugs/show_bug.cgi?id=5117

This commit just adds a sceleton without any functionality.

If anybody is interested to learn about polyhedral optimizations this would be
a good task. Well definined, self contained and pretty simple. Ping me if you
want to start and you need some pointers to get going.

llvm-svn: 149386