[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>!

llvm-svn: 255247