[DeLICM] Add Knowledge class. NFC.

The Knowledge class remembers the state of data at any timepoint of a SCoP's
execution. Currently, it tracks whether an array element is unused or is
occupied by some value, and the writes to it. A future addition will be to also
remember which value it contains.

Objects are used to determine whether two Knowledge contain conflicting
information, i.e. two states cannot be true a the same time.

This commit was extracted from the DeLICM algorithm at
https://reviews.llvm.org/D24716.

llvm-svn: 295197