Use WeakVH instead of WeakTrackingVH in AliasSetTracker's UnkownInsts

In cases where an instruction (a call site, say) is RAUW'ed with some
other value (this is possible via the `returned` attribute, for
instance), 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.

This relands r301426.

llvm-svn: 301814