[LICM] Allow promotion of some stores that are not guaranteed to execute.

Promotion is always legal when a store within the loop is guaranteed to execute.

However, this is not a necessary condition - for promotion to be memory model
semantics-preserving, it is enough to have a store that dominates every exit
block. This is because if the store dominates every exit block, the fact the
exit block was executed implies the original store was executed as well.

Differential Revision: https://reviews.llvm.org/D28147

llvm-svn: 291171