[AArch64] Ensure no tagged memory is left in the unallocated portion of the

stack

This patch makes sure that if we tag some memory, we untag that memory before
the function returns/throws via any exit, reachable from the tag operation. For
that we place the untag operation either at:

  a) the lifetime end call for the alloca, if that call post-dominates the
     lifetime start call (where the tag operation is placed), or it (the
     lifetime end call) dominates all reachable exits, otherwise
  b) at the reachable exits

Differential Revision: https://reviews.llvm.org/D68469

llvm-svn: 374182