[DSE] Eliminate stores at the end of the function.

This patch add support for eliminating MemoryDefs that do not have any
aliasing users, which indicates that there are no reads/writes to the
memory location until the end of the function.

To eliminate such defs, we have to ensure that the underlying object is
not visible in the caller and does not escape via returning. We need a
separate check for that, as InvisibleToCaller does not consider returns.

Reviewers: dmgreen, rnk, efriedma, bryant, asbirlea, Tyker, george.burgess.iv

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D72631