Scalar: Ignore ConstantData in processAssumption

Assumptions on UndefValue and ConstantPointerNull aren't relevant to
other users.  Ignore them entirely to avoid wasting cycles walking
through their (possibly extremely extensive (cross-module)) use-lists.

It wasn't clear how to add a specific test for this, and it'll be
covered anyway by an eventual patch that asserts when trying to access
the use-list of an instance of ConstantData.

llvm-svn: 282334