[MemoryDependency] Relax the re-ordering with volatile store.

Volatile store does not provide any special rules for reordering with
atomics. Usual must alias anaylsis is enough here.

This makes the bahavior similar to how volatile load is handled.

Reviewers: reames, nikic
Reviewed By: reames
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D119818