[MTE] [HWASan] Support diamond lifetimes.

We were overly conservative and required a ret statement to be dominated
completely be a single lifetime.end marker. This is quite restrictive
and leads to two problems:

* limits coverage of use-after-scope, as we degenerate to
  use-after-return;
* increases stack usage in programs, as we have to remove all lifetime
  markers if we degenerate to use-after-return, which prevents
  reuse of stack slots by the stack coloring algorithm.

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D127905