[DeadStoreElimination] Add support for non-local DSE.

[DeadStoreElimination] Add support for non-local DSE.

We extend the search for redundant stores to predecessor blocks that
unconditionally lead to the block BB with the current store instruction.  That
also includes single-block loops that unconditionally lead to BB, and
if-then-else blocks where then- and else-blocks unconditionally lead to BB.

http://reviews.llvm.org/D13363
Patch by Ivan Baev <ibaev@codeaurora.org>!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255247 91177308-0d34-0410-b5e6-96231b3b80d8