Use a WeakVH for UnknownInstructions in AliasSetTracker

Summary:
This change solves the same problem as D30726, except that this only
throws out the bathwater.

AST was not correctly tracking and deleting UnknownInstructions via
handles.  The existing code only tracks "pointers" in its
`ASTCallbackVH`, so an UnknownInstruction (that isn't also def'ing a
pointer used by another memory instruction) never gets a
`ASTCallbackVH`.

There are two other ways to solve this problem:

 - Use the `PointerRec` scheme for both known and unknown instructions.
 - Use a `CallbackVH` that erases the offending Instruction from the
   UnknownInstruction list.

Both of the above changes seemed to be significantly (and unnecessarily
IMO) more complex than this.

Reviewers: chandlerc, dberlin, hfinkel, reames

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D30849

llvm-svn: 297539