[Attributor][OpenMP] Add assumption for non-call assembly instructions

Inline assembly is scary but we need to support it for the OpenMP GPU
device runtime. The new assumption expresses the fact that it may not
have call semantics, that is, it will not call another function but
simply perform an operation or side-effect. This is important for
reachability in the presence of inline assembly.

Differential Revision: https://reviews.llvm.org/D109986