Improve filechecking of volatile test.

My previous attempt was extremely deficient, allowing more volatiles
to be introduced and not even checking all of the ones that are
present.

This attempt doesn't try to keep track of the values stored or offsets
within particular objects, just that the correct objects are accessed
in a correctly volatile manner throughout.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174700 91177308-0d34-0410-b5e6-96231b3b80d8