[Instruction] Introduce getAtomicSyncScopeID()

An analysis may just be interested in checking if an instruction is
atomic but system scoped or single-thread scoped, like ThreadSanitizer's
isAtomic(). Unfortunately Instruction::isAtomic() can only answer the
"atomic" part of the question, but to also check scope becomes rather
verbose.

To simplify and reduce redundancy, introduce a common helper
getAtomicSyncScopeID() which returns the scope of an atomic operation.
Start using it in ThreadSanitizer.

NFCI.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D121910