The absence of noreturn doesn't ensure mayReturn

The absence of noreturn doesn't ensure mayReturn

There are two separate issues:
- LLVM doesn't consider infinite loops to be side effects: we happily
  hoist/sink above/below loops whose bounds are unknown.
- The absence of the noreturn attribute is insufficient for us to know
  if a function will definitely return.  Relying on noreturn in the
  middle-end for any property is an accident waiting to happen.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273762 91177308-0d34-0410-b5e6-96231b3b80d8