[x86] fix 256-bit vector store splitting to honor 'volatile'

Forking this out of the discussion in D62498
(and assuming that will be committed later, so adding the helper function here).
The LangRef says:
"the backend should never split or merge target-legal volatile load/store instructions."

Differential Revision: https://reviews.llvm.org/D62506

llvm-svn: 361815