[Hexagon] Misaligned loads and stores are not fast

The DAG combiner tries to merge stores to adjacent vector wide memory
locations by creating stores which are integral multiples of the vector
width. Discourage this by informing it that this is slow. This should
not affect legalization passes, because all of them ignore the "Fast"
argument.

Patch by Pranav Bhandarkar.

llvm-svn: 277178