[DSE,MSSA] Eliminate stores by terminators (free,lifetime.end).

This patch adds support for eliminating stores by free & lifetime.end
calls. We can remove stores that are not read before calling a memory
terminator and we can eliminate all stores after a memory terminator
until we see a new lifetime.start. The second case seems to not really
trigger much in practice though.

Reviewers: dmgreen, rnk, efriedma, bryant, asbirlea, Tyker

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D72410