DeadStoreElimination can now trim the size of a store if the end of it is dead.

DeadStoreElimination can now trim the size of a store if the end of it is dead.

Only currently done if the later store is writing to a power of 2 address or
has the same alignment as the earlier store as then its likely to not break up
large stores into smaller ones

Fixes <rdar://problem/10140300>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143630 91177308-0d34-0410-b5e6-96231b3b80d8