[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.

llvm-svn: 277182