[DeLICM] Known knowledge.

Extend the Knowledge class to store information about the contents
of array elements and which values are written. Two knowledges do
not conflict the known content is the same. The content information
if computed from writes to and loads from the array elements, and
represented by "ValInst": isl spaces that compare equal if the value
represented is the same.

Differential Revision: https://reviews.llvm.org/D31247

llvm-svn: 302339