Support atomic write operations in stack safety

This has two benefits:
    * we can now mark allocas that are used in atomic operations as safe
    * this fixes a bug that would incorrectly mark all atomic writes as safe
      in HWASan instrumentation. this is because stack safety keeps a list
      of all *unsafe* operations that are reachable from an alloca, but it
      did not analyze atomic writes, so it would always mark them as safe.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D159153