[FunctionAttrs] Extract a helper function for the core logic used to

[FunctionAttrs] Extract a helper function for the core logic used to
evaluate whether 'readonly' or 'readnone' apply to a given function.
This both reduces indentation and will make it easy to share the logic
with a new pass manager implementation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248181 91177308-0d34-0410-b5e6-96231b3b80d8