[SparsePropagation] Enable interprocedural analysis

This patch adds the ability to perform IPSCCP-like interprocedural analysis to
the generic sparse propagation solver. The patch gives clients the ability to
define their own custom LatticeKey types that the generic solver maps to custom
LatticeVal types. The custom lattice keys can be used, for example, to
distinguish among mappings for regular values, values returned from functions,
and values stored in global variables. Clients are responsible for defining how
to convert between LatticeKeys and LLVM Values by providing a specialization of
the LatticeKeyInfo template.

The added unit tests demonstrate how the generic solver can be used to perform
a simplified version of interprocedural constant propagation.

Differential Revision: https://reviews.llvm.org/D37353

llvm-svn: 315919