[Assignment Tracking] Fix mem2reg misidentifying unlinked stores

updateForDeletedStore updates the assignment tracking debug info for a store
that is about to be deleted by mem2reg. For each variable backed by the target
alloca, if a dbg.assign exists it is kept (well - it's downgraded to a
dbg.value). A dbg.value is inserted if there's not a linked dbg.assign for a
variable which is backed by the target alloca. This patch fixes a bug whereby a
store with a linked dbg.assign that describes a fragment different to the one
linked to the alloca was not counted for the variable, leading to both keeping
the dbg.assign (downgrading it) and inserting a new dbg.value.

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D146299