[FunctionAttrs] Factor the logic to test for a known non-null return out

of a method and into a re-usable static helper. We can potentially use
this function from the implementation of a new pass manager oriented
version of the pass. Also add some better documentation of exactly what
the semantic model of this routine is (it isn't trivial) and use a more
modern naming convention for it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247524 91177308-0d34-0410-b5e6-96231b3b80d8