[CFLAA] Teach CFLAnders to distinguish reads from writes.

This patch adds more specific edges to CFLAndersAliasAnalysis. The goal
of these edges is to give us more information about *how* two values
that MayAlias alias. With this, we can now tell cases like

a = b; // ergo, a may alias b

apart from

a = c;
b = c;

// so, a may alias b, but only because they were both assigned to c.

...And others.

Patch by Jia Chen.

Differential Revision: https://reviews.llvm.org/D22429

llvm-svn: 276023