Use WeakVH instead of WeakTrackingVH in AliasSetTracker's UnkownInsts

Summary:
In cases where an instruction (a call site, say) is RAUW'ed with some
other value (this is possible via the `returned` attribute, amongst
other things), we want the slot in UnknownInsts to point to the
original Instruction we wanted to track, not the value it got replaced
by.

Fixes PR32587.

Reviewers: davide

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D32268

llvm-svn: 301426