[CFLAA] Simplify CFLGraphBuilder. NFC.

This patch simplifies the graph builder by encoding nodes as {Value,
Dereference Level} pairs. This lets us kill edge types, and allows us to
get rid of hacks in StratifiedSets (like addAttrsBelow/...). This
simplification also allows us to remove InstantiatedRelations and
InstantiatedAttrs.

Patch by Jia Chen.

Differential Revision: http://reviews.llvm.org/D22080

llvm-svn: 275122