[Hexagon] Testcase for not merging stores into a misaligned store

[Hexagon] Testcase for not merging stores into a misaligned store

The DAG combiner will try to merge consecutive stores into a bigger
store, unless the resulting store is not fast. Misaligned vector stores
are allowed on Hexagon, but are not fast. Add a testcase to make sure
this type of merging does not occur.

Patch by Pranav Bhandarkar.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@277182 91177308-0d34-0410-b5e6-96231b3b80d8